Preheat the oven to 400°F. Arrange the bell peppers cut side up in a baking dish, drizzle lightly with olive oil, season with salt, and bake for 10 minutes to soften slightly.

While the peppers bake, heat the remaining olive oil and butter in a large skillet over medium-high heat. Add the sliced onion and mushrooms and cook, stirring often, until softened and starting to brown.

Stir in the optional red pepper strips and garlic, cooking for 1 to 2 minutes until fragrant. Add the Italian seasoning, Worcestershire sauce, salt, and pepper, then transfer the veggie mixture to a plate.

In the same skillet, add the sliced steak and cook just until no longer pink, about 3 to 4 minutes, keeping the heat fairly high so it sears instead of steams.

Return the veggie mixture to the skillet, toss with the steak, and adjust the seasoning to taste. The filling should be juicy but not soupy.

Remove the par-baked peppers from the oven and place half a slice of cheese in the bottom of each. Spoon the hot steak and veggie mixture into the peppers, packing it in evenly.

Top each pepper with the remaining cheese slices and return the baking dish to the oven. Bake for 10 to 15 minutes, until the peppers are tender and the cheese is melted and lightly golden.

Let the Philly cheesesteak stuffed peppers rest for a few minutes, then sprinkle with parsley and serve warm.