Cut the tops off the bell peppers and remove the seeds and membranes. Lightly coat the outside with olive oil and place them upright in a baking dish.
In a large skillet over medium heat, add the sausage. Cook until browned and cooked through, breaking it apart with a spatula.
Add the minced garlic to the skillet and cook for an additional 1 minute until fragrant.
Stir in the chopped spinach and cook until wilted, about 2-3 minutes. Remove from heat.
In a large mixing bowl, combine the cooked sausage mixture, ricotta cheese, cream cheese, mozzarella cheese, Parmesan cheese, Italian seasoning, salt, pepper, and red pepper flakes (if using). Mix until well combined.
Spoon the creamy sausage and spinach filling evenly into each prepared bell pepper.
Cover the baking dish tightly with aluminum foil and bake in the preheated oven for 25 minutes.
Remove the foil and sprinkle extra mozzarella cheese on top of each stuffed pepper.
Bake uncovered for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven, let them cool for a few minutes, and serve warm.
Notes
Feel free to use chicken sausage for a lighter option.