Make the topping: Combine the cheddar, panko, chives, and melted butter in a small bowl and rub the mixture together. Set aside.

Make  the dip: Melt the butter in a medium skillet over medium-low heat. Add the flour and whisk until the mixture the is bubbly and just starting to color, about 4 minutes. Pour in the milk and sherry, if using, and bring to a simmer, whisking to keep the sauce from sticking. (Don't forget the corners of the pan!)

Heat the broiler, on low, if that's an option, with a rack in the top position. If your broiler doesn't have a low setting, bring the rack down a level. Butter a 4-cup flameproof casserole dish

and set it aside.

Remove the  sauce from the heat and stir in the lemon juice. Gently fold in the crabmeat until it's evenly coated with sauce. Pour the mixture into the prepared casserole dish and sprinkle the topping evenly over it. Broil until the topping is golden brown, about 4 minutes. Let stand for a few minutes before serving with the crackers or toasted baguette.