Step 1: Prep the Shrimp

1. Preheat your oven to 400°F (200°C).
2. Rinse the shrimp under cold water and pat dry with paper towels. Season with a pinch of salt and black pepper.

Step 2: Make the Garlic Cream Sauce

1. In a medium-sized skillet, melt 2 tablespoons of butter over medium heat.
2. Add the minced garlic and chopped onion. Sauté until fragrant and the onions are translucent, about 3-4 minutes.
3. Pour in the heavy cream, stirring constantly. Allow it to simmer for 2-3 minutes until slightly thickened.
4. Stir in the Parmesan and mozzarella cheeses, mixing until the cheeses melt and the sauce becomes smooth and creamy.
5. Remove from heat and adjust seasoning with salt and pepper as needed.

Step 3: Assemble the Gratin

1. Spread the shrimp evenly in a shallow baking dish or gratin dish.
2. Pour the garlic cream sauce over the shrimp, ensuring each piece is coated.

Step 4: Prepare the Breadcrumb Topping

1. In a small bowl, combine the breadcrumbs, smoked paprika, and chopped parsley.
2. Melt the remaining 2 tablespoons of butter and drizzle it over the breadcrumb mixture. Toss until evenly coated.

Step 5: Bake the Gratin

1. Sprinkle the breadcrumb topping evenly over the shrimp and sauce.
2. Place the baking dish in the preheated oven and bake for 15-20 minutes, or until the shrimp are pink and opaque, and the topping is golden brown.
3. For an extra-crispy topping, switch to the broiler setting for the last 2 minutes of cooking.