Giant Portobello Burgers with Caramelized Onions
For the caramelized onions
  1. Caramelize the onions: Peel, halve, and thinly slice the onions. In a very large skillet, heat the olive oil over medium-high heat. Add the onions, which may just fit the skillet and can be mounded if necessary. Saute, stirring frequently, for about 10 minutes until soft and translucent.
  2. Add the kosher salt and reduce the heat to medium low, finding a temperature that is not so hot that the onions burn but not so low that they do not continue to cook. Cook slowly, stirring every 5 to 10 minutes or so, until reduced and dark brown, about 40 minutes. The onions will turn from translucent white to golden to a rich, dark brown color, and the volume will reduce by at least four times.
  3. Grill the mushrooms and peppers: Preheat a grill to medium high (for an alternative roasting method, see Notes).
  4. Brush any dirt from the mushroom caps and remove the stems. Place the mushrooms gill-side up on a baking sheet. Peel and mince the garlic. Sprinkle the garlic onto the mushrooms, then drizzle each cap with about ½ tablespoon olive oil and 1 teaspoon balsamic vinegar. Add a generous pinch of kosher salt to each (about ½ teaspoon in total) and several grinds of black pepper to each mushroom.
  5. For the bell peppers, turn each pepper on its side and cut off top. Remove pith and seeds, then slice the pepper along its cut side into 1/4-inch rings. Slice the cheese.
  6. Place the mushrooms gill side up on the hot grill over direct heat. Place the pepper rings along the edges of the grill over indirect heat. Grill the mushrooms until tender and juicy, about 12 to 15 minutes, then flip, draining any juices, and grill an additional 2 minutes. If desired, flip one more time and melt sliced cheese onto the mushrooms. Meanwhile, cook the peppers until tender, about 15 minutes. If desired, place the peppers over direct heat toward the end of the cooking time to blacken them.
  7. To serve, slice the avocado, tomato, and red onion and wash the arugula. Serve the portobellos on buns topped with grilled peppers, avocado, tomato, red onion, arugula, and caramelized onions.
Recipe Notes

Alternative method: Preheat the oven to 450°F. Line a rimmed baking sheet with parchment paper or a silicone mat. Place the mushrooms gill-side up on the sheet and prepare with garlic, olive oil, balsamic vinegar, and salt as instructed. Add 1/2 teaspoon liquid smoke to each mushroom. Slice the bell peppers as noted, then place them on the same baking sheet. Roast for 15 minutes until tender. Drain of any excess liquid as necessary.