In a large skillet over medium-high heat, add 1 tablespoon of vegetable oil. Sauté the sliced green pepper, red pepper, and onion until they’re tender and slightly caramelized, about 5–7 minutes. Push vegetables to one side of the skillet (or remove and set aside).
Cook the Steak:
Add a bit more oil to the empty side of the skillet. Add the shaved sirloin steak and season with salt, pepper, and garlic powder. Cook until the meat is browned and fully cooked through, about 6–8 minutes. Set meat aside with the vegetables if removed.
Make the Sauce:
Reduce the heat to medium. In the same skillet, melt the butter, then pour in the heavy whipping cream. Stir occasionally until the cream begins to gently simmer.
Add Cheese:
Gradually stir in the Parmesan and mozzarella cheeses. Continue stirring until the cheese is fully melted and the sauce is smooth and creamy.
Combine Everything:
Return the cooked steak and sautéed vegetables to the skillet, stirring to combine. Add the cooked pasta and gently toss everything together until well coated.
Serve:
Serve warm and enjoy immediately. Garnish with additional Parmesan or fresh herbs if desired.