Preheat the oven to 375°F (190°C).
Lightly grease a small baking dish or oven-safe skillet.
Mix the base. In a bowl, beat the cream cheese until smooth. Stir in mayo, sour cream, Old Bay, Dijon, Worcestershire, lemon juice, and garlic if using.
Fold in cheese and scallions. Add cheddar, half the Parmesan, and scallions. Taste and adjust seasoning with salt, pepper, or more Old Bay.
Add the crab gently. Fold in the lump crab with a light hand to keep the chunks intact.
Don’t overmix.
Transfer and top. Spread the mixture into your baking dish. Sprinkle the remaining Parmesan on top.
Bake 18–22 minutes, until hot and bubbling around the edges and lightly golden on top. For extra color, broil 1–2 minutes at the end, watching closely.
Rest and serve. Let it sit 5 minutes.
Finish with a pinch of Old Bay and a squeeze of lemon if you like. Serve warm with toasts and crackers.