Heat a drizzle of olive oil in a large pot or Dutch oven over medium heat. Add the sliced smoked sausage and cook for about 5 minutes until browned. Remove the sausage and set aside.
In the same pot, add the diced onion and bell pepper. Sauté for about 3-4 minutes until softened. Then, add the minced garlic and cook for an additional minute until fragrant.
Return the smoked sausage to the pot. Add the penne pasta, diced tomatoes (with their juice), chicken broth, Italian seasoning, and red pepper flakes (if using). Stir well to combine.
Bring the mixture to a boil. Once boiling, reduce the heat to medium-low and cover. Cook for about 10-12 minutes, or until the pasta is al dente, stirring occasionally to prevent sticking.
When the pasta is cooked, stir in the fresh spinach and cook for another 2-3 minutes until wilted. Season with salt and pepper to taste.
Remove from heat and sprinkle the shredded cheese on top. Cover for a few minutes until the cheese melts.
Serve warm, garnished with additional cheese if desired.