Grilled Portobello mushroom burgers

These Portobello mushroom burgers get beautifully caramelized by marinating mushrooms and onions before grilling. With a super easy pesto mayonnaise, peppery arugula, and roasted peppers, these burgers will be a summertime hit.

It’s grilling season! While grilling is often where meat recipes shine, I love to have a good repertoire of vegetarian recipes for the grill as well. These Portobello mushroom burgers are every bit as hearty and satisfying as a beef burger or steak sandwich and still scream summertime. They are super easy to throw together – with a flavorful marinade and pesto mayonnaise relying heavily on pantry ingredients and store-bought shortcuts – and only a few fresh ingredients.

How to make grilled Portobello mushroom burgers

The key to these burgers is marinating both the mushrooms and the onions, adding a little sweetness from balsamic vinegar to help with caramelization and bumping up the umami with a bit of soy sauce. To take it over the top, you make a super easy, three-ingredient pesto mayo, which pairs perfectly with some peppery arugula and refreshing (storebought!) roasted peppers.

I have only tested these on a gas grill; however, a charcoal grill or an indoor grill pan should work as well. You want to maintain a medium to medium-low heat. You are not looking to char the vegetables – the marinade will help them caramelize at relatively low temps. On my gas grill, medium-low (with a temp around 400 F – 425 F) worked perfectly for me.

Tips and substitutions for grilled Portobello mushroom burgers

• Go vegan or gluten-free. You can easily make these burgers vegan and gluten-free with just a few substitutions. To go vegan, swap the provolone for a non-dairy cheese or simply leave it out, and use a vegan mayonnaise. To go gluten free, use a gluten-free burger bun and a gluten-free soy sauce or tamari.
• Grill however you can. I’ve only tested these on a gas grill, but you could also use a charcoal grill or indoor grill pan. Just don’t use too high of temps – because of the marinade, high temps will char the mushrooms and onions rather than caramelizing them.
• Save your leftovers! I can say with full confidence that these burgers heat up well (unassembled, of course). Additionally, you will likely have extra pesto mayonnaise. Keep it and slather on EVERYTHING.

Grilled Portobello mushroom burgers

These Portobello mushroom burgers get beautifully caramelized by marinating mushrooms and onions before grilling. With a super easy pesto mayonnaise, peppery arugula, and roasted peppers, these burgers will be a summertime hit.

Ingredients

Burgers

4 Portobello mushrooms

1 large red onion or sweet onion

4 slices provolone cheese

1 or 2 roasted red peppers, torn into strips (I use jarred, but homemade also works)

2 cups arugula

4 burger buns (any will do, but I like brioche or challah buns)

Marinade

3 Tablespoons balsamic vinegar

1 Tablespoon soy sauce

1 teaspoon Dijon mustard

1/2 teaspoon salt

1/2 teaspoon herb seasoning blend (like Trader Joe’s 21 Seasoning Salute, or an Italian herb blend)

several grinds black pepper

2 Tablespoons olive oil

Pesto mayonnaise

1/2 cup mayonnaise

2 Tablespoons pesto (storebought or homemade)

2 teaspoons Dijon mustard

salt & pepper

Directions

1. Clean the mushrooms by wiping with a damp paper towel and remove the stems. Cut off the root and tip of the onion, peel it, and then slice into about 1/2 inch circles. Place the mushrooms and onion slices on a sheet pan.
2. Make the marinade by combining all the ingredients in a small bowl and whisk well. Brush the marinade onto the mushrooms and onions on both sides. Finish with the mushrooms bottom-side up, letting excess marinade pool into the middle of the mushroom and soak in. Allow to sit at room temperature for 30-60 minutes.
3. While the mushrooms marinate, make the pesto mayonnaise. Stir together the mayonnaise, pesto, and Dijon mustard, then season with salt and pepper to taste. Store in the refrigerator until you are ready to assemble the burgers.
4. When the marinating time is almost up, get the grill ready. For a gas grill, preheat to a medium-low setting, with a temp around 400 F. For a charcoal grill, allow the coals to ash over fully before adding the mushrooms and onions, or heap more to the edges or one side, if your grill is big enough. You don’t want a high heat or the vegetables will burn before cooking through.
5. Add the mushrooms and onions to the grill, starting the mushrooms with the top down. Grill the mushrooms 5-8 minutes per side and the onions 8-10 minutes per side, moving as needed to prevent charring. In the last minute of grilling, add the provolone cheese (if using) on top of the mushrooms to melt slightly. Before turning the grill off, toast the buns on the grill, if desired.
6. To assemble the burgers, spread both sides of the bun with the pesto mayonnaise, then top with arugula, Portobello mushrooms, grilled onions, and roasted pepper strips. Enjoy!