Heat the olive oil in a large pot or Dutch oven over medium-high heat. Add the sliced flank steak and season with salt and pepper. Sear the steak for about 3-4 minutes until browned, then remove it from the pot and set aside.
In the same pot, add the chopped onion, bell pepper, and minced garlic. Sauté for about 3-4 minutes until the vegetables are softened.
Stir in the diced tomatoes (with their juice), beef broth, Italian seasoning, and red pepper flakes. Bring the mixture to a simmer.
Add the penne pasta to the pot, stirring well to combine. Cook according to package instructions, stirring occasionally, for about 10-12 minutes, or until the pasta is al dente.
Once the pasta is cooked, return the seared steak to the pot. Mix well and let it simmer for an additional 2-3 minutes until heated through.
Taste and adjust the seasoning with salt and pepper, if necessary.
Serve hot, garnished with fresh parsley and grated Parmesan cheese if desired.