Step 1

Coat the mushroom and onion with oil and sprinkle with salt and thyme, to taste.

Step 2

Bring a grill pan (or large skillet) sprayed with nonstick spray to medium-high heat on the stove. Place the mushroom in the pan, round side down, along with the onion, side by side. Cook for 5 minutes without flipping.

Step 3

Meanwhile, pull string cheese into pieces and set aside. Season the mayo with cayenne pepper, to taste; mix well and set aside.

Step 4

Flip the mushroom and onion in the pan. Top the mushroom evenly with cheese, and cook for another 5 minutes, or until cheese and veggies have softened. Remove from the pan and set aside.

Step 5

Split apart the bun and spread the inside of each half with butter. Sprinkle with garlic powder. Place the bun halves in the pan, still over medium-high heat, with the buttered sides down. (Alternatively, toast bun halves in a toaster oven.) Once warm and toasty, after 1 to 2 minutes, remove and plate with the buttered sides up.

Step 6

Spread the buttered side of the bun's top half with mayo, and set aside. Place the mushroom on the bottom half of the bun; top with onion, tomato, and lettuce. Finish off your sandwich with the top half of the bun. Now CHEW!