01 - Preheat oven to 375°F. Slice the tops off the bell peppers and remove the seeds and membranes. Lightly brush the outside of each pepper with olive oil.
02 - In a skillet over medium heat, add 1 tablespoon olive oil and sauté the chopped spinach for 2–3 minutes until wilted. Set aside to cool slightly.
03 - In a large bowl, combine the ricotta cheese, 1/4 cup Parmesan, egg, sautéed spinach, garlic, basil, oregano, nutmeg (if using), salt, and pepper. Mix until fully incorporated.
04 - Divide the ricotta mixture evenly among the bell peppers, packing it in gently. 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 stuffed peppers with the mozzarella and the remaining 1/4 cup Parmesan cheese.
07 - Cover the 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 rest for 5 minutes before serving.