Preheat oven to 375°F (190°C) and lightly grease a casserole dish.
Melt butter in a skillet over medium heat.
Add chopped onion and cook until soft.
Stir in minced garlic and cook for about 30 seconds.
Add heavy cream and mayonnaise and mix until smooth.
Stir in paprika, Old Bay seasoning, salt, and black pepper.
Add shrimp and crab meat and gently mix to combine.
Pour the mixture into the prepared casserole dish.
Sprinkle cheddar and mozzarella cheese evenly over the top.
Add breadcrumbs for a crispy topping.
Bake for 20–25 minutes until the casserole is bubbly and golden.
Drizzle lemon juice over the casserole before serving.
Garnish with chopped parsley.