This vegetarian baked spaghetti casserole combines pasta, a savory tomato sauce, vegetables, and plenty of melted cheese in one comforting oven-baked dish. It is easy to assemble with familiar ingredients, and baking lets the flavors come together while the top turns golden and bubbly.
Ingredients
- 12 ounces (340 g) spaghetti
- 1 tablespoon olive oil
- 1 medium yellow onion, finely chopped
- 1 medium zucchini, diced small
- 8 ounces (225 g) cremini or white mushrooms, chopped
- 3 cloves garlic, minced
- 1 teaspoon dried Italian seasoning
- 1/2 teaspoon fine salt, plus more for the pasta water
- 1/4 teaspoon black pepper
- 1 jar (24 ounces/680 g) marinara sauce
- 1 can (14.5 ounces/411 g) diced tomatoes, drained
- 1/2 cup (120 g) ricotta cheese
- 1 large egg
- 2 cups (200 g) shredded mozzarella, divided
- 1/2 cup (50 g) finely grated Parmesan cheese, divided (use vegetarian Parmesan if needed)
- 2 tablespoons chopped fresh parsley, optional
Instructions
- Heat the oven and prepare the dish. Heat the oven to 375°F (190°C). Lightly grease a 9-by-13-inch baking dish. Bring a large pot of water to a boil and salt it generously.
- Cook the spaghetti. Break the spaghetti in half if you prefer easier-to-serve pieces, then cook it for 2 minutes less than the package’s al dente time. It will continue cooking in the oven. Drain well; do not rinse, since the starch helps the sauce cling.
- Cook the vegetables. While the pasta cooks, warm the olive oil in a large skillet over medium heat. Add the onion, zucchini, and mushrooms. Cook for 7–9 minutes, stirring occasionally, until softened and most of the liquid released by the mushrooms has evaporated. Add the garlic, Italian seasoning, salt, and pepper; cook for 30 seconds.
- Make the tomato mixture. Stir in the marinara and drained diced tomatoes. Bring the mixture to a gentle simmer, then remove the skillet from the heat. Taste and adjust the seasoning if needed.
- Mix the creamy layer. In a medium bowl, stir together the ricotta, egg, 1 cup of the mozzarella, and 1/4 cup of the Parmesan until combined. The egg helps the ricotta mixture set into the casserole; mix it away from the hot skillet so it does not scramble.
- Combine and layer. Add the drained spaghetti to the skillet, or transfer everything to a large bowl if the skillet is too small. Toss until coated with sauce. Spread half of the spaghetti mixture in the baking dish. Spoon the ricotta mixture over it in small dollops and gently spread as evenly as possible. Add the remaining spaghetti, then top with the remaining 1 cup mozzarella and 1/4 cup Parmesan.
- Bake until bubbly. Cover the dish loosely with foil and bake for 20 minutes. Remove the foil and bake for another 10–15 minutes, until the sauce bubbles around the edges and the cheese is melted and lightly browned. For a deeper golden top, broil briefly, watching closely so it does not burn.
- Rest and serve. Let the casserole stand for 10 minutes before cutting. This helps it hold together when served. Sprinkle with parsley, if using.
Tips
- Cook the spaghetti just shy of al dente. Fully tender pasta can become soft after the additional time in the oven.
- Let the mushrooms release their moisture and cook it off before adding sauce. Excess liquid can make the casserole watery.
- For a meatless dish, check that the marinara and cheeses are vegetarian. Some Parmesan-style cheeses use animal rennet; look for microbial or vegetarian rennet where that matters to you.
- If your baking dish is particularly full, place it on a rimmed sheet pan to catch any bubbling sauce. Use oven mitts when removing the hot dish.
Serving Suggestions
Serve warm squares with a crisp green salad dressed with lemon vinaigrette, or pair the casserole with steamed broccoli or green beans. Garlic bread is a welcome side for soaking up extra sauce. Since the pasta and cheese make this a substantial main dish, keep sides simple and fresh. For a family meal, set out extra grated Parmesan, black pepper, and chopped parsley so everyone can finish their portion to taste.
Variations
Use chopped spinach in place of the zucchini, or add a diced red bell pepper with the onion. If using spinach, wilt it in the skillet and squeeze out excess water if it was frozen. For more protein, fold in one drained 15-ounce can of cannellini beans or chickpeas with the sauce. You can replace ricotta with cottage cheese, blended briefly for a smoother texture, or use a dairy-free ricotta and mozzarella for a plant-based version; check that the egg is also replaced with a suitable binder if needed. For a spicier casserole, add 1/4 teaspoon red pepper flakes with the garlic.
Storage and Reheating
Refrigerate leftovers in a covered, shallow container within 2 hours of cooking. Keep them at 40°F (4°C) or colder and use within 3–4 days. Reheat individual portions in the microwave until steaming hot throughout, stirring or turning as needed for even heating. To reheat a larger portion, cover it with foil and warm in a 350°F (175°C) oven until hot in the center, usually 20–30 minutes depending on its size. Food safety guidance is to reheat leftovers to 165°F (74°C); check the center with a food thermometer if you are unsure. Avoid reheating the same portion more than once. Freeze fully cooled portions in airtight, freezer-safe containers for up to 2–3 months; thaw overnight in the refrigerator before reheating. The texture of the pasta may soften slightly after freezing.
Frequently Asked Questions
Can I assemble the casserole ahead of time?
Yes. Assemble it, cover, and refrigerate for up to 24 hours before baking. Because it will be cold, allow extra oven time and bake until the center is hot and the cheese is bubbling. For food safety, keep it refrigerated until it goes into the oven rather than leaving it at room temperature.
Can I use a different pasta shape?
Yes. Penne, rigatoni, or another sturdy pasta works well. Use 12 ounces and cook it about 2 minutes short of the package’s al dente time. The casserole layers most easily with long spaghetti, but shorter shapes are convenient to scoop and serve.
Why did my casserole turn out watery?
The vegetables may have released too much moisture, or the diced tomatoes may not have been drained. Cook the mushrooms and zucchini until their excess liquid evaporates, drain the tomatoes, and drain the pasta thoroughly. A 10-minute rest after baking also helps the sauce settle.
Can I make it without the egg?
Yes. The egg helps the ricotta layer set, but it is not essential. Leave it out for a softer, creamier result, or use a prepared egg replacer according to its package directions. Avoid adding extra liquid to the ricotta mixture.
How can I tell when it is done?
The cheese should be melted and lightly golden, and the sauce should bubble at the edges. If the casserole was refrigerated before baking, check that the middle is steaming hot; a food thermometer should read 165°F (74°C) for reheated leftovers.
Can I freeze the casserole before baking?
Yes. Assemble it in a freezer-safe dish, wrap it tightly, and freeze for up to 2–3 months. Thaw it in the refrigerator before baking for the most even result. Do not put a frozen glass dish directly into a hot oven, since sudden temperature changes can cause it to break.