Preheat the oven: Set to 375°F (190°C). Lightly grease a small baking dish (8x8 or similar).
Cook the shrimp: Heat butter or oil in a skillet over medium.
Add shrimp, a pinch of salt and pepper, and cook 2–3 minutes until just pink. Remove from heat and cool slightly.
Mix the creamy base: In a large bowl, beat cream cheese until smooth. Stir in sour cream, mayonnaise, Dijon, lemon juice, garlic, Old Bay, and crushed red pepper (if using).
Mix until well combined.
Fold in cheese and aromatics: Add half of the mozzarella and half of the Parmesan, plus green onions and parsley. Stir gently.
Add the seafood: Fold in the chopped shrimp and lump crab. Taste and adjust with more lemon, Old Bay, salt, or pepper.
Aim for a bright, savory flavor.
Assemble the dip: Spread the mixture evenly in the baking dish. Top with the remaining mozzarella and Parmesan.
Bake: Bake 18–22 minutes until hot and bubbling at the edges and the top is lightly golden. If you want extra color, broil 1–2 minutes, watching closely.
Garnish and serve: Sprinkle with extra parsley and sliced green onion.
Serve hot with your favorite dippers.