Preheat the oven to 375°F. Butter a shallow 11⁄2-quart casserole.
In a skillet over medium heat, melt the butter. Add the onion, stir, and cook for 5 minutes, or until soft. Add the garlic and seafood seasoning, stir to blend, and cook for 1 minute more. Set aside.
Combine the cream cheese, sour cream, and mayonnaise in a bowl and, using an electric mixer on medium-low speed, mix until just blended. Add the lemon juice, Worcestershire sauce, and mustard and mix until smooth and creamy. Add salt and pepper, to taste. Fold in the crabmeat and 1 cup of the cheese.
Spread the mixture in the prepared casserole. Sprinkle with the remaining 1 cup of cheese and dust with paprika. Bake for 25 minutes, or until bubbly. Serve warm.