01 - Preheat oven to 375°F. Slice the tops off the bell peppers and remove all seeds and membranes. Lightly brush the outside of each pepper with olive oil.
02 - Heat 1 tablespoon of olive oil in a skillet over medium heat. Add the chopped spinach and sauté for 2 to 3 minutes until wilted. Set aside to cool slightly.
03 - In a large mixing bowl, combine the ricotta cheese, 1/4 cup of Parmesan, egg, sautéed spinach, minced garlic, basil, oregano, nutmeg (if using), salt, and pepper. Mix until all ingredients are fully incorporated.
04 - Fill each bell pepper evenly with the ricotta mixture and place them upright in a baking dish.
05 - Spoon the tomato sauce around the base of the peppers in the baking dish.
06 - Sprinkle the stuffed peppers with shredded mozzarella and the remaining 1/4 cup of Parmesan cheese.
07 - Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and continue baking for another 10 minutes until the peppers are tender and the cheese is golden and bubbly.
08 - Remove from the oven and let the peppers rest for 5 minutes before serving.