1. Preheat oven to 400 degrees. Melt 6 tablespoons butter in a medium saucepan over medium heat. Add onion and garlic; cook, stirring occasionally, until soft, about 4 minutes. Whisk in flour; cook, whisking constantly, 4 minutes. Continue to whisk; pour in milk in a slow, steady stream. Simmer over medium heat, whisking, until thickened, about 4 minutes. Stir in cayenne and mustard. Gradually whisk in cheese; whisk until melted.

2. Remove from heat. Stir in lemon juice, zest, and Worcestershire. Stir in crabmeat and parsley. Season with salt and pepper. Transfer to a 1-quart ovenproof dish.

3. Melt remaining 2 tablespoons butter; stir into bread pieces. Season with salt and pepper. Arrange bread mixture over crab dip. Bake until heated through and golden brown, 25 to 30 minutes. Let stand at room temperature 10 minutes. Serve with baguette slices.