Combine onion, bell pepper, oil and 1 teaspoon seasoning in a mixing bowl. Stir to evenly coat vegetables with oil and seasoning. Using grill pan, grill with direct medium heat (350°F – 450°F) 8-10 minutes until vegetables are tender and browned. Remove from grill and cover until burgers are done.
Gently shape ground beef into 5-inch burgers. Season burgers on both sides with 1 tablespoon seasoning.
Grill over direct medium heat (with the lid closed as much as possible), until the internal temperature reaches 160°F for medium, 12-15 minutes, turning once. Top each with 1 slice of cheese just before removing from the grill. Place buns on grill and toast for 1-2 minutes.
Place burgers on toasted buns, top with onion and pepper mixture, Weber® Sweet & Thick Original BBQ Sauce, and second slice of cheese.
Serve with pepperoncini, if desired.
When you’re grilling up burgers, don’t forget the Weber® Gourmet Burger™ Seasoning.