Step 1

Rinse/dry or gently wipe off the mushrooms with a damp paper towel. Trim the stem so the mushroom cap can rest flat. With a fork, prick top of mushroom cap several times.

Step 2

In a small bowl, whisk together the balsamic, olive oil, mustard, oregano, cumin, salt, and pepper.

Step 3

Add the mushrooms to a large ziploc bag. Pour in the marinade. Close and shake everything around to completely coat the mushrooms. Set in fridge for 30 minutes.

Step 4

Heat 2 tbsp. olive oil in a large frying pan on medium. Add the onion and garlic. Cook for 12-15 minutes, stirring regularly until golden and fragrant.

Step 5

Line a baking sheet with foil. Turn the oven to broil and place the oven rack in the upper third (but not directly under the heating element).

Step 6

Place the mushrooms gill side down on the baking sheet. Cook for 5 minutes under the broiler. Flip and cook for another 4-5 minutes until barely starting to char.

Step 7

Slice the buns. For best results drizzle a bit of olive oil and toast for a minute or two under the broiler.

Step 8

Place the cooked portabello cap on the bun and top with fresh arugula (or greens of choice), onion, condiments of choice, and bun.