Preheat oven to 400 degrees
Heat a large skillet over medium-high heat, Add the beef and cook until no longer pink. Add the onion, garlic, red pepper flakes, red pepper flakes, and Italian seasoning and cook for about 1 more minute.
Remove beef mixture from skillet and transfer into large bowl.
Add the spinach to the skillet and cook until wilted. Drain any excess water from the spinach. Add the spinach to the beef mixture.
Add the parmesan and ricotta cheese to the bowl. Mix until well combined,. Set aside.
Remove the seeds and stems from the bell peppers, slicing the peppers into 3 or 4 flat pieces. Line a cast iron skillet (or baking dish) with the peppers.
Add the beef mixture on top of the peppers in an even layer.
Add the tomato sauce on top of the stuffed peppers.
Sprinkle the mozzarella cheese on top of the tomato sauce layer.
Bake in the oven for about 30 minutes, then top with fresh basil.