01 - Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
02 - Boil salted water in a large pot and cook the pasta until al dente. Drain thoroughly and set aside.
03 - Melt butter in a large skillet over medium heat. Add the chopped onion and minced garlic, sautéing until fragrant, approximately 2 minutes.
04 - Add the shrimp to the skillet and cook until they turn pink, about 2–3 minutes. Remove the shrimp from the skillet and set aside.
05 - Add the broccoli florets to the same skillet. Sauté for 2–3 minutes until they are bright green and slightly tender.
06 - Pour the heavy cream into the skillet and bring it to a gentle simmer. Stir in 1 cup of grated Parmesan cheese and the 1 cup of shredded mozzarella cheese until fully melted and the sauce is smooth. Season with salt, black pepper, and nutmeg if desired.
07 - Add the cooked pasta, sautéed broccoli, and cooked shrimp to the sauce in the skillet. Gently toss to ensure all components are well coated.
08 - Transfer the combined mixture to the prepared baking dish. Evenly sprinkle the top with the remaining 1/2 cup of grated Parmesan cheese.
09 - Bake uncovered in the preheated oven for 20 minutes, or until the top is golden brown and the casserole is bubbling.
10 - Remove the casserole from the oven. Sprinkle with fresh chopped parsley and allow it to rest for 5 minutes before serving.