Step 1

Preheat oven to 400F

Step 2

Brush mushrooms clean (do not wet or wash) with a brush or dry sponge and carefully remove stems. Brush or spray with olive oil and generously season with salt and fresh black pepper on both sides. Place on a sheet pan, gill-side down.  Bake for 10 minutes (if they are extra large, add a few more minutes) then turn them over. Cook 5-7 minutes, and add cheese to the gill side if using, letting it melt for the final few minutes.

Step 3

Make the quick pickled onions and Chipotle Mayo- whisking everything in a small bowl.

Step 4

When ready to assemble- toast the buns. You can do this in a toaster, or on a skillet with a little olive oil.

Step 5

Assemble the burgers: Spread the buns (top and bottom) with the Chipotle Mayo, place the portobello mushroom on the buns, add sliced tomato (season with salt and pepper), add avocado, (season with salt and pepper), top arugula and pickled onions. Top with the top bun.

Step 6

Serve immediately!