Prepare the Rice
First, cook your rice if you don’t already have leftovers. Arborio rice works best because of its creamy texture, but any short-grain rice will do. Make sure the rice is not overcooked as it will continue to bake in the oven.
Cook the Sausage and Veggies
In a large skillet, heat the olive oil over medium heat. Add chopped onions and garlic, and sauté for 2-3 minutes until fragrant and translucent. If you're using sausage, now is the time to cook it. Crumble the sausage into the skillet and cook until browned. If you’re going for a vegetarian option, skip this step or substitute with diced mushrooms.
Mix Everything Together
In a large mixing bowl, combine the cooked rice, sausage mixture (or mushrooms), tomato sauce, oregano, salt, pepper, eggs, half of the mozzarella, and half of the Parmesan. Stir everything together until well mixed.
Layer the Riso al Forno
Preheat your oven to 375°F (190°C). In a greased baking dish, pour half of the rice mixture. Sprinkle a layer of mozzarella on top, then pour the rest of the rice mixture. Top with the remaining mozzarella and Parmesan.
Bake
Place the dish in the preheated oven and bake for 35-40 minutes, or until the top is golden and bubbly. Let it rest for 10 minutes before serving to allow the flavors to meld together.
Garnish and Serve
Once out of the oven, garnish with fresh basil if you like, and serve warm.