Preheat your grill to medium-high heat. Be sure you have a clean grate and wipe it down with a little oil when hot.
In a large bowl, mix ground meat, egg, chopped onion and mushrooms.
Sprinkle in black pepper and garlic salt as you’re mixing the meat.
Form small burgers with hands (no need for a fancy slider mold here). The burgers will be about 2 inches in diameter.
Slice all three peppers down the sides to form large flat pieces (think like you’re cutting the walls off a house).
Slice other onion and tomatoes thinly (about 1/8 inch). Pull lettuce apart into individual leaves. Try to keep them whole (as large as possible).
Grill burgers 3-5 minutes per side, depending upon your desired doneness.
Grill peppers and mushrooms 2-3 minutes per side, or until cooked through.