In the same skillet, add the diced onion, green bell pepper, and mushrooms (if using). Cook for 5-6 minutes, stirring occasionally, until the vegetables are soft and the onions are translucent. Add the minced garlic and cook for another minute, until fragrant.
Make the Sauce
Pour in the beef broth, scraping the bottom of the skillet to release any flavorful browned bits. Lower the heat, then add the heavy cream. Stir to combine, bringing the mixture to a gentle simmer. The sauce should begin to thicken slightly, coating the back of a spoon.
Add Cheese and Beef
Return the cooked beef to the skillet, along with the shredded provolone (or mozzarella) and cheddar cheese. Stir until the cheese melts and the sauce becomes creamy and smooth. The sauce should be rich and cheesy at this point.
Combine with Pasta
Add the drained pasta to the skillet, stirring to coat each piece with the creamy beef and cheese mixture. Taste and adjust seasoning with salt and pepper if needed.
