In a large pot of salted boiling water, cook the ditalini pasta according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add the Italian sausage and break it apart with a wooden spoon. Cook until browned and cooked through, about 5-7 minutes.
Stir in the chopped onion and garlic, cooking until the onion becomes translucent, approximately 3-4 minutes. Then add the diced red bell pepper and cook for an additional 2-3 minutes until soft.
Add in the diced tomatoes (with juice), chicken broth, dried oregano, red pepper flakes (if using), salt, and pepper. Stir well and bring to a simmer. Let simmer for about 5 minutes to allow flavors to meld.
Add the cooked ditalini pasta and fresh spinach to the skillet. Stir gently to combine, allowing the spinach to wilt into the warm pasta and sauce, about 2 minutes.
Remove the skillet from heat and mix in the grated Parmesan cheese until well incorporated. Adjust seasoning with additional salt and pepper as needed.
Divide the pasta among plates or serve in a large family-style bowl. Garnish with fresh basil leaves and extra Parmesan if desired.
Notes
Feel free to adjust the spice level by choosing mild or spicy sausage.