Step 1

Preheat the oven to 425 degrees F. Line 2 rimmed baking sheets with parchment and set aside.

Step 2

Roast the mushrooms: Arrange the mushrooms, stem side up, onions and garlic cloves in a single layer on 1 prepared baking sheet. Drizzle with half of the olive oil. Drizzle 1/2 cup water around the mushrooms on the baking sheet. Season generously with salt and pepper. Roast in the center of the oven until the mushrooms shrink in size and become darker brown, 15 to 18 minutes. Remove and let cool.

Step 3

Bread the mushrooms: In a medium bowl, whisk the eggs with a splash of water. In another medium bowl, combine the breadcrumbs with the granulated garlic. Dip each mushroom in the egg mixture and then coat completely with the breadcrumbs. Reserve everything.

Step 4

Fry the mushrooms: In a large skillet, heat the remaining olive oil over medium heat until it begins to smoke. Add the mushrooms in a single layer. Cook on one side until golden brown, 2 to 3 minutes. Flip and cook on the second side until golden brown, another 2 to 3 minutes. Remove the mushrooms from the oil and arrange in a single layer on a clean baking sheet. Season with salt and pepper.

Step 5

Make the filling: In a medium bowl, combine the roasted garlic cloves and onion slices with the cheese. Add any remaining egg and 1/4 cup of the breadcrumbs leftover from the breading. Add the basil and mix well.

Step 6

Assemble and bake: On the same baking sheet, place 2 portobellos, stem-side up, with some distance between them. Pile half of the filling on each and then top with the remaining mushrooms, pressing gently but firmly down, so it forms a fairly compact sandwich. Bake until the cheese melts, 6 to 8 minutes.

Step 7

Arrange the lettuce to form 2 cups. Drizzle some pickle juice and red wine vinegar into the cups. Season with salt and pepper and then top with some sliced pickles.

Step 8

Place each mushroom burger onto a bottom bun. Flip a lettuce cup onto each burger and top with the top bun. Serve.