Heat oven to 400°F. Line a sheet pan with foil.
Split the bread lengthwise. Set cut sides up on the pan.
Stir mayonnaise with 1 teaspoon garlic and a pinch of salt.
Warm olive oil and butter in a large skillet over medium-high heat.
Soften onions and peppers for 5–6 minutes. Add mushrooms; cook until browned.
Push vegetables aside. Sear steak in the same pan until just cooked through.
Season with remaining garlic, Worcestershire, salt, and pepper. Toss to combine.
Spread garlic mayo over bread. Mound steak and veggies evenly on top.
Cover with provolone. Bake until melted and edges crisp, 10–12 minutes.
Broil 1–2 minutes for extra color, if desired. Rest briefly.
Finish with parsley and red pepper flakes. Slice and serve hot.