In a medium bowl, combine ground beef, A1 sauce, garlic powder, salt, and pepper (I use my hands to mix it all together). Shape into 2–4 equal patties, placing a small indentation in the middle of each patty. Set aside.
Heat olive oil in a large skillet over medium-high heat. Add peppers and onions and saute until veggies are crisp-tender. Add mushrooms and cook until mushrooms are browned. Remove from heat.
Place burgers on grill. Cook, covered, for 4 minutes. Flip burgers and grill, covered, for an additional 4 minutes. Top each patty with 2 slices of cheese and close grill lid. Leave burgers on grill until cheese is melted to your liking. Remove from grill.
Place patties on hamburger buns, top each with bell pepper mixture and pepperoncinis.
Recipe by Simple, Sweet & Savory at https://www.simplesweetsavory.com/philly-cheesesteak-burgers/