Slice the tops off the bell peppers and carefully remove the seeds and membranes. Set aside.
In a large skillet, heat the olive oil over medium heat. Add the chopped spinach and sauté for about 2-3 minutes until wilted.
In a mixing bowl, combine the sautéed spinach, ricotta cheese, half of the mozzarella cheese, grated Parmesan cheese, garlic powder, onion powder, salt, pepper, and crushed red pepper flakes (if using). Mix well until fully combined.
Stuff each bell pepper generously with the ricotta and spinach mixture. Place stuffed peppers upright in a baking dish.
Top each pepper with the remaining mozzarella cheese.
Cover the baking dish with foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10-15 minutes until the cheese is melted and bubbly.
Once cooked, remove from the oven and let cool for a few minutes. Garnish with fresh basil leaves before serving.
Notes
Feel free to add other vegetables or spices to the filling for extra flavor.