Preheat the oven. Set to 375°F (190°C).
Lightly grease a small baking dish (about 1–1.5 quarts) for an even bake.
Sauté the shrimp. Warm a skillet over medium heat with butter or oil. Add shrimp with a pinch of salt and pepper and cook 1–2 minutes per side, just until pink. Remove from heat and let cool slightly, then chop into bite-size pieces if not already chopped.
Prep the creamy base. In a mixing bowl, beat together cream cheese, sour cream, and mayonnaise until smooth.
A hand mixer makes this easy, but a sturdy spoon works too.
Fold in flavor. Stir in garlic, green onions, lemon juice, Old Bay, smoked paprika, cayenne, and a pinch of salt and pepper. Mix until evenly combined.
Add the cheese. Fold in mozzarella and Parmesan, reserving a tablespoon or two of Parmesan for topping. The mixture should be thick and spreadable.
Stir in the seafood. Gently fold in crab and the cooked shrimp.
Try not to break up the crab too much; those big flakes are delicious.
Taste and adjust. Add a little more lemon, salt, or seasoning as needed. The mixture should taste slightly bold now, since flavors mellow as it bakes.
Spread and top. Transfer to your greased baking dish, smooth the top, and sprinkle with the reserved Parmesan. Add a light dusting of paprika if you want extra color.
Bake. Bake for 18–22 minutes, until hot and bubbling around the edges.
For a deeper golden top, broil for 1–2 minutes at the end, watching closely.
Finish and serve. Let it rest 5 minutes to settle, then garnish with chopped parsley. Serve warm with your favorite dippers.