Coat the mushroom and onion with oil and sprinkle with salt and thyme, to taste.
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.
Meanwhile, pull string cheese into pieces and set aside. Season the mayo with cayenne pepper, to taste; mix well and set aside.
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.
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.
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!
PER SERVING (entire recipe): 263 calories, 10.25g fat, 786mg sodium, 32.5g carbs, 7.5g fiber, 7g sugars, 15.5g protein