Preheat your oven to 375°F and coat a 9x13-inch baking dish with nonstick spray or a drizzle of olive oil.

Bring a large pot of salted water to a rolling boil. Add the orzo and cook according to package directions until al dente. Drain and set aside.

In a large skillet, heat the olive oil over medium heat. Add the Italian sausage, breaking it into smaller pieces as it cooks. Cook for 5 to 7 minutes until browned.

Add the chopped onion and minced garlic to the skillet. Sauté for 3 to 4 minutes until the onion is soft. Stir in the diced tomatoes, chopped spinach, Italian seasoning, salt, and black pepper. Cook for another 2 to 3 minutes.

Transfer the sausage and vegetable mixture into a large mixing bowl. Add the cooked orzo and stir everything until coated. Fold in half of the mozzarella and half of the Parmesan cheese.

Spoon the mixture into the prepared baking dish, spreading it evenly. Sprinkle the remaining mozzarella and Parmesan cheese on top.

Bake in the preheated oven for 20 to 25 minutes until the top is golden and bubbly. If it's browning too fast, tent it with foil.

Let the dish rest for about 5 minutes before serving. Top with fresh basil if using.