Steps
1. Cook the pasta according to the package instructions until al dente.
2. In a skillet, melt the butter over medium heat.
3. Add the sliced onions and sauté until translucent, about 3-4 minutes.
4. Add the sliced beef to the skillet and cook until browned, about 5-6 minutes.
5. Stir in the sliced bell pepper and cook for an additional 3-4 minutes until tender.
6. Drain the cooked pasta and add it to the skillet, mixing well.
7. Season with salt and pepper to taste. Serve hot, garnished with Parmesan cheese if desired.