This grilled onions and mushrooms recipe turns two humble ingredients into a deeply caramelized, savory side that pairs with almost anything. The high, direct heat of the grill caramelizes the natural sugars in the onions while the mushrooms soak up a simple garlic-herb butter, creating a rich, smoky flavor in under 20 minutes. It’s an easy, hands-off side that works on any grill or grill pan, and it’s the perfect topping for steaks, burgers, or sandwiches.
Ingredients
- 2 large yellow or sweet onions (about 1 lb / 450 g total), peeled and cut into 1/2-inch (1.25 cm) thick rounds
- 1 lb (450 g) cremini or button mushrooms, cleaned and stems trimmed (leave small ones whole, halve or quarter larger ones so they’re uniform)
- 3 tbsp (45 ml) olive oil, divided
- 2 tbsp (28 g) unsalted butter, melted
- 2 cloves garlic, minced
- 1 tbsp (15 ml) balsamic vinegar (optional, for brightness)
- 1 tsp (5 g) kosher salt, plus more to taste
- 1/2 tsp (2 g) black pepper
- 1 tbsp (3 g) fresh thyme leaves (or 1 tsp dried thyme)
- 2 tbsp (8 g) chopped fresh parsley, for garnish (optional)
Instructions
- Preheat your grill to medium-high heat (about 400°F / 200°C). If using a charcoal grill, let the coals burn until they’re covered with gray ash.
- In a small bowl, mix the melted butter, minced garlic, and thyme. Set aside.
- In a large bowl, toss the onion rounds with 1 1/2 tablespoons (22 ml) of the olive oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper. In a separate bowl, toss the mushrooms with the remaining 1 1/2 tablespoons (22 ml) olive oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper.
- Place the onion rounds directly on the grill grate. Grill for 4–5 minutes per side, until they have nice grill marks and are starting to soften. Transfer to a plate or a sheet of foil.
- Place the mushrooms on the grill. If using a grill basket, use that to prevent them from falling through the grates. Grill for 6–8 minutes, turning occasionally, until they are browned and tender.
- Return the onions to the grill (or keep them on a warm part of the grill) and brush both the onions and mushrooms with the garlic-thyme butter. Grill for another 1–2 minutes, until the butter is melted and fragrant.
- Remove everything to a serving platter. Drizzle with balsamic vinegar if using, and sprinkle with fresh parsley. Season with additional salt and pepper to taste. Serve hot.
Tips
- Keep onion rounds intact: Thread a toothpick or small skewer through each onion round horizontally to keep the rings from separating on the grill. Remove before serving.
- Don’t crowd the mushrooms: If you pile too many mushrooms on the grill, they’ll steam instead of brown. Cook them in a single layer, working in batches if needed.
- Use a grill basket for small mushrooms: If you’re using small button mushrooms, a grill basket or a piece of heavy-duty foil with holes poked in it will save you from losing them through the grates.
- Substitutions: No butter? Use all olive oil. No fresh thyme? Use 1 teaspoon dried. You can also swap balsamic vinegar for red wine vinegar or a squeeze of lemon juice.
Serving Suggestions
Serve these grilled onions and mushrooms as a side to grilled steak, chicken, or pork chops. They’re also excellent piled on top of burgers, folded into an omelet, or layered into a grilled cheese sandwich. For a vegetarian main, serve them over polenta, mashed potatoes, or crusty bread with a dollop of ricotta or goat cheese.
Variations
- Balsamic-Glazed: Add 2 tablespoons (30 ml) balsamic vinegar to the butter mixture and brush it on during the last 2 minutes of grilling for a sweet-tangy glaze.
- Spicy: Add 1/4 teaspoon red pepper flakes or a dash of hot sauce to the butter mixture.
- Herb-Forward: Use rosemary or oregano instead of thyme, and add a teaspoon of Dijon mustard to the butter.
- Smoky: Sprinkle with 1/2 teaspoon smoked paprika before grilling.
Storage and Reheating
Store leftovers in an airtight container in the refrigerator for up to 4 days. To reheat, warm them in a skillet over medium heat for 2–3 minutes, or microwave in 30-second bursts until hot. You can also freeze them in a single layer on a baking sheet, then transfer to a zip-top bag for up to 3 months; thaw overnight in the fridge before reheating.
Frequently Asked Questions
Can I use a grill pan instead of an outdoor grill? Yes, a cast-iron grill pan works great. Heat it over medium-high heat for 5 minutes, then cook the onions and mushrooms in batches to avoid overcrowding. You may need to add a little more oil to prevent sticking.
Should I clean the mushrooms before grilling? Yes, but don’t soak them. Wipe each mushroom with a damp paper towel or give them a quick rinse and pat dry thoroughly. Excess water will cause them to steam rather than brown.
How do I keep the onions from falling apart? Cutting the onions into thick rounds (about 1/2 inch) and using a toothpick or skewer through the side will help hold the rings together. Also, let them cook undisturbed for the first few minutes so they release from the grill naturally.
Can I prepare these ahead of time? You can slice the onions and clean the mushrooms up to a day ahead, storing them in separate airtight containers in the fridge. The butter mixture can also be made ahead and refrigerated for up to 3 days.
What’s the best mushroom variety? Cremini (baby bella) mushrooms are ideal because they’re meaty and hold up well on the grill. White button mushrooms work too, but they’re smaller and cook faster—check them at the 5-minute mark. Portobello caps can be grilled whole and sliced after.
Is it safe to grill onions and mushrooms directly on the grates? Yes, as long as the pieces are large enough not to fall through. For small mushrooms, use a grill basket or skewer them. Onion rounds are fine directly on the grate, but a grill mat or foil works if you prefer.