Cut the beef into thin strips against the grain and prepare the green and red bell peppers, onion, and garlic by slicing and mincing respectively. Cook the tortellini according to the package instructions until tender but firm; drain and keep aside.
Warm the olive oil in a large skillet over medium-high heat. Quickly brown the beef strips for 2 to 3 minutes, seasoning with salt and pepper. Remove the beef and set it aside. Using the same skillet, cook the sliced onions and bell peppers until they soften, about 5 to 7 minutes. Add the garlic and cook for an additional minute. Return the seared beef to the pan and mix everything well; set aside.
In a medium pot, whisk together the flour with ¼ cup cold milk to make a smooth slurry. Heat the remaining 1¼ cups milk and beef broth over medium heat until warm, then slowly whisk in the flour mixture. Keep stirring for 5 to 7 minutes until the sauce thickens and barely begins to bubble. Lower the heat and stir in the shredded provolone cheese, along with salt and pepper, until the cheese melts fully and the sauce remains smooth.
Combine the cooked tortellini and beef-vegetable mixture with the creamy provolone sauce, folding gently to mix everything. Serve the dish right away topped with chopped fresh parsley for a pop of color. For a richer option, place the mixture in a baking dish, sprinkle extra cheese on top, and bake at 375°F (190°C) for 15 to 20 minutes until bubbly and golden brown.
