Portobello Mushroom Burger with Balsamic Onions (Grilled Vegetarian Burger)

How to Make Portobello Mushroom Burger with Balsamic Onions (Grilled Vegetarian Burger)

1. 1

   Step 1: Make the balsamic onions

   Thinly slice the red onion and heat 1 tablespoon of olive oil in a skillet over medium heat until shimmering. Add the onions and a pinch of kosher salt and black pepper, cooking until soft and golden, then stir in 3 tablespoons balsamic vinegar and the brown sugar and simmer until the mixture is jammy, about 6–8 minutes.

   For this step

   • 4 tbspBalsamic vinegar
   • 1 largeRed onion
   • 1 tbspBrown sugar
   • 1 tspKosher salt
   • 1/2 tspFreshly ground black pepper
   
2. 2

   Step 2: Prepare and marinate the mushrooms

   Wipe the portobello caps clean and remove the stems, then whisk together the remaining 3 tablespoons olive oil, 1 tablespoon balsamic vinegar, minced garlic, and a pinch of salt and pepper. Brush the mixture over both sides of the mushrooms and let them sit to marinate while the onions finish, about 10 minutes.

   For this step

   • 2 clovesGarlic
   
3. 3

   Step 3: Preheat grill and grill the mushrooms

   Preheat the grill to medium-high and oil the grates to prevent sticking, then place the mushrooms gill-side down and grill for 4–5 minutes. Flip and grill another 3–4 minutes, basting with any remaining marinade, until mushrooms are tender and nicely charred.
4. 4

   Step 4: Melt the cheese

   During the final minute of grilling, place a slice of provolone on each mushroom cap and close the grill lid to melt the cheese gently. Remove the mushrooms when the cheese is melted and glossy, transferring them to a clean plate to rest for a minute.

   For this step

   • 4 slicesProvolone cheese
   
5. 5

   Step 5: Toast buns and assemble burgers

   Toast the burger buns cut-side down on the grill for 20–30 seconds until lightly charred and warmed through. Layer each bottom bun with a grilled mushroom, top with a generous spoonful of balsamic onions, then finish with the bun top and serve immediately.

   For this step

   • 4 bunsBurger buns
   
6. 6

   Step 6: Rest and serve

   Let assembled burgers rest for a minute so the juices settle and the cheese sets slightly. Serve warm with your favorite sides and an extra drizzle of balsamic if desired.

1. Thinly slice the red onion and heat 1 tablespoon of olive oil in a skillet over medium heat until shimmering. Add the onions and a pinch of kosher salt and black pepper, cooking until soft and golden, then stir in 3 tablespoons balsamic vinegar and the brown sugar and simmer until the mixture is jammy, about 6–8 minutes.
2. Wipe the portobello caps clean and remove the stems, then whisk together the remaining 3 tablespoons olive oil, 1 tablespoon balsamic vinegar, minced garlic, and a pinch of salt and pepper. Brush the mixture over both sides of the mushrooms and let them sit to marinate while the onions finish, about 10 minutes.
3. Preheat the grill to medium-high and oil the grates to prevent sticking, then place the mushrooms gill-side down and grill for 4–5 minutes. Flip and grill another 3–4 minutes, basting with any remaining marinade, until mushrooms are tender and nicely charred.
4. During the final minute of grilling, place a slice of provolone on each mushroom cap and close the grill lid to melt the cheese gently. Remove the mushrooms when the cheese is melted and glossy, transferring them to a clean plate to rest for a minute.
5. Toast the burger buns cut-side down on the grill for 20–30 seconds until lightly charred and warmed through. Layer each bottom bun with a grilled mushroom, top with a generous spoonful of balsamic onions, then finish with the bun top and serve immediately.
6. Let assembled burgers rest for a minute so the juices settle and the cheese sets slightly. Serve warm with your favorite sides and an extra drizzle of balsamic if desired.