Deviled Crab with Sherry Sauce

1/2 stick (1/4 cup) unsalted butter

2 tablespoons all-purpose flour

whole milk

1 large garlic clove, lightly crushed

2 large egg yolks

1/3 cup medium-dry Sherry

1/2 teaspoon dry mustard

1/8 teaspoon freshly grated nutmeg

1/8 teaspoon cayenne

1/2 teaspoon salt

2 tablespoons finely chopped fresh flat-leaf parsley

1/4 cup fine dry bread crumbs

1/4 cup finely grated Parmigiano -Reggiano (1/2 oz)