Prepare the pasta according to the directions on the package. Once cooked, drain and set aside.
While the pasta cooks, heat the oil in a skillet until it's hot. Add the sirloin and cook for about 5 minutes, or until it's nicely browned. Remove the beef from the skillet and cover loosely with foil. Dispose of any excess grease from the skillet but don’t clean it.
In a large skillet, melt the butter. Add the diced onions, green pepper, mushrooms, and garlic. Season with salt and black pepper, and cook until the vegetables are tender, which should take around 5 minutes.
Whisk in the flour, then gradually stir in the half-and-half and whole milk. Let the mixture come to a simmer. Incorporate the provolone cheese, stirring until fully melted. Add the cooked pasta shells to the cheese sauce and mix thoroughly.
Finally, stir in the cooked sirloin. Serve and enjoy!