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. Stir until well incorporated.
04 - Divide the ricotta mixture evenly among the prepared bell peppers, filling each one generously. Place the stuffed peppers upright in a baking dish.
05 - Spoon the tomato sauce around the base of the peppers in the baking dish.
06 - Sprinkle the shredded mozzarella and the remaining 1/4 cup of Parmesan evenly over the top of each stuffed pepper.
07 - Cover the baking dish tightly with aluminum foil and bake for 25 minutes.
08 - Remove the foil and continue baking for 10 minutes until the peppers are tender and the cheese is golden and bubbly. Let rest for 5 minutes before serving.