Prep the bread. Preheat your oven to 400°F (200°C). Line a baking sheet with foil. Slice the loaf lengthwise and place both halves cut-side up on the sheet.
Make the garlic butter. In a small bowl, mix softened butter, minced garlic, parsley, a pinch of salt, and a dash of black pepper.
Spread evenly over both bread halves.
Toast the bread base. Bake the bread for 6–8 minutes until lightly golden at the edges. This helps it stay crisp under the toppings.
Cook the veggies. Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add onions and peppers with a pinch of salt.
Cook, stirring, for 6–8 minutes until softened and lightly caramelized. Transfer to a bowl.
Sear the steak. Add another tablespoon of oil to the skillet. Increase heat to high.
Season steak with salt, pepper, and onion powder. Sear in batches for 1–2 minutes per side until just cooked and still tender. Don’t overcrowd the pan.
Combine. Return veggies to the skillet with the steak.
Toss together. If using Cheez Whiz or American cheese, stir in a few spoonfuls now to coat the mixture and make it extra creamy.
Layer the bread. Remove toasted bread from the oven. Lay provolone slices across both halves, edge to edge.
Pile the steak-and-veggie mix on top, spreading evenly. Sprinkle mozzarella over everything. Add crushed red pepper flakes if you like heat.
Final bake. Return to the oven for 8–10 minutes, until the cheese is fully melted and bubbly.
For extra browning, switch to broil for the last 1–2 minutes, watching closely.
Finish and serve. Let rest 2–3 minutes. Slice into thick pieces. Serve hot, with napkins ready.