Preheat the oven: Start by preheating your oven to 375°F (191°C). This is crucial for ensuring that your peppers cook evenly.
Prepare the peppers: Place the halved peppers skin side down in a baking dish. Bake for 10 minutes, then flip them over and bake for another 5-10 minutes until tender. This process softens the peppers and ensures they have a nice texture when they are served.
Sauté the vegetables: Meanwhile, heat 1 tablespoon of olive oil in a skillet over medium heat. Add the diced onion and minced garlic, sautéing until both become translucent and fragrant, usually about 3-4 minutes.
Add the spinach: Stir in the ½ teaspoon kosher salt, ½ teaspoon black pepper, and ⅛ teaspoon ground nutmeg. Then, add the 3 cups of fresh baby spinach. Cook for 1-2 minutes until the spinach wilts down significantly, reducing its volume.
Combine the filling: Off the heat, mix in the 12 oz ricotta cheese and 1 ½ cups of shredded mozzarella cheese until everything is well combined and creamy.
Stuff the peppers: Carefully fill each baked pepper with the spinach and cheese mixture, pressing down gently to pack it in. Top each stuffed pepper with the remaining 1 cup of shredded mozzarella cheese for that delicious, cheesy topping.
Bake the stuffed peppers: Place the stuffed peppers back in the oven and bake for an additional 8-10 minutes, or until the cheese is melty and bubbly.
Serve: Once finished baking, let the stuffed peppers cool for 5-10 minutes. This makes them easier to handle and allows the flavors to meld better. Serve warm and enjoy!