Tray of Balsamic Marinated Portobello Burgers topped with Caramelized Onions and Garlic Aioli
4.45 from 20 votes

Balsamic Portobello Burgers with Caramelized Onions & Garlic Aioli

These 10-ingredient portobello burgers are the best mushroom burgers we've ever had! It’s the balsamic-herb marinade and homemade garlic aioli that sets them apart. Sure to please plant-based and meat eaters alike!

Author: Minimalist Baker
Prep Time 1 hour 15 minutes
Cook Time 10 minutes
Total Time 1 hour 25 minutes
Servings: 4 (Burgers)
Category: Entree
Cuisine: American, Vegan
Freezer Friendly No
Does it keep? 2-3 Days



  • 1 head garlic (to roast)
  • 3/4 cup raw cashews
  • 2 cloves garlic, minced (omit for less intense garlic flavor)
  • 1 Tbsp avocado or olive oil
  • 1/3 cup water (or sub unsweetened almond milk for creamier consistency)
  • Salt + pepper (to taste)


  • 4 medium-large portobello mushrooms (organic when possible)
  • 3 Tbsp avocado or olive oil
  • 1/4 cup balsamic vinegar
  • 1 Tbsp Italian seasoning (dried basil + oregano)
  • 2 cloves garlic, minced (2 cloves yield ~1 Tbsp or 6 g)
  • 1 healthy pinch each salt and black pepper


  • 1 Tbsp avocado or olive oil
  • 1 medium white or yellow onion, peeled + sliced into 1/4-inch rings


  • 4 whole wheat hamburger buns (ensure vegan-friendly as needed // sub lettuce wraps or GF buns for gluten-free)
  • 2 ripe tomatoes, sliced (optional // organic when possible)
  • Lettuce or greens of choice (optional)


  1. Preheat oven to 400 degrees F (204 C) and cut off the very top of a head of garlic. Drizzle with a little oil, sprinkle with sea salt, and loosely wrap in foil. Set directly on an oven rack and bake for 1 hour. Let cool before use. Note: To save time, you could buy roasted garlic, or sub the head of roasted garlic for 3-4 additional cloves raw garlic and proceed as instructed.

  2. In the meantime, place cashews in a bowl and cover with just boiling water. Let sit uncovered for 1 hour. Then drain and rinse thoroughly.

  3. While cashews are soaking, use a damp washcloth or paper towel to clean the mushrooms by wiping away any dirt and removing the stems. Place stem side up on a rimmed baking sheet or 9x13-inch baking dish.

  4. To a small mixing bowl, add avocado oil, balsamic vinegar, Italian seasoning, minced garlic, salt, and pepper. Whisk to combine, then pour over the mushrooms and use a pastry brush to distribute the marinade. Flip the mushrooms over and brush the other side, then flip back over to stem-side-up position. Set aside, uncovered at room temperature, to marinate (at least 30 minutes).

  5. Add your drained, soaked cashews to a small blender. Take your roasted head of garlic and press out the softened garlic by squeezing from the base up until all of the garlic is extracted.

  6. To the blender also add fresh garlic, avocado oil, water (or almond milk), and a pinch each salt and pepper. Blend on high until creamy and smooth, scraping down sides as needed. Taste and adjust seasonings as needed, adding more olive oil or water for creaminess or salt for saltiness. Transfer to a serving dish and set aside.

  7. To caramelize onions, heat a large skillet over medium heat. Once hot, add 1-2 Tbsp avocado oil, the sliced onion, and a pinch of salt. Sauté, stirring frequently, until light golden brown and very soft (see photo). Remove from skillet and set aside.

  8. Return the skillet to the stovetop (or heat a grill and lightly oil grilling surface). Once hot, add portobello mushrooms — only as many as will fit comfortably in the pan — and sauté for about 3-6 minutes on each side, or until the mushrooms are deep golden brown on the edges and softened/reduced in size (see photo). While cooking, brush with any leftover marinade for extra flavor.

  9. At this time, prepare any serving items, such as browning hamburger buns on the skillet (or grill), slicing tomato, or chopping greens.
  10. To serve, add a generous spoonful of garlic aioli to each bun and top with aioli, then tomato, lettuce (or greens), mushrooms, onions, and more aioli on top of the bun. 

  11. Enjoy immediately. Store leftover mushrooms separately in the refrigerator up to 2-3 days, though best when fresh. Aioli will keep in the refrigerator for 7-10 days.


*Recipe adapted from our 31 Meals Cookbook.
*Nutrition information is a rough estimate calculated with avocado oil and without optional ingredients.

Nutrition Per Serving (1 of 4 burgers)