Prep the oven and pan: Heat the oven to 425°F (220°C). Lightly grease a 9x13-inch baking dish with cooking spray or a little oil.

Roast the cauliflower: Toss the florets with 1 tablespoon olive oil, half the salt, and half the pepper. Spread on a sheet pan in a single layer. Roast 18–22 minutes, until edges are lightly browned and the florets are just tender. Set aside.

Sauté peppers and onions: In a large skillet over medium-high heat, add the remaining 1 tablespoon olive oil and the butter. Add the sliced onion and bell peppers with a pinch of salt. Cook 7–9 minutes, stirring occasionally, until softened and lightly caramelized. Stir in the minced garlic and cook 30 seconds until fragrant. Transfer to a bowl.

Cook the beef: In the same skillet, add the sliced beef in a single layer. Season with remaining salt, pepper, onion powder, and garlic powder. Cook 2–4 minutes, just until browned and no longer pink. Don’t overcook—it will bake again. Stir in Worcestershire, if using, and turn off the heat.

Combine in the baking dish: Add roasted cauliflower to the prepared baking dish. Top evenly with the pepper-onion mixture and the cooked beef. Toss gently to distribute.

Add the cheese: Sprinkle mozzarella all over, then layer or scatter provolone on top. If you like a little heat, add a pinch of red pepper flakes.

Bake: Reduce oven to 400°F (205°C). Bake 12–15 minutes, until the cheese is melted and bubbly. For extra browning, broil for 1–2 minutes at the end, watching closely.

Finish and serve: Let the casserole rest 5 minutes. Garnish with chopped parsley. Taste and add a final sprinkle of salt if needed. Serve warm.