Spread 1 slice of bread with mustard and top with pickles, meats, and cheese. Mince and mash garlic to a paste with a pinch of salt, the mix with mayonnaise. Spread on remaining slice of bread and assemble sandwich.

Beat together egg, milk, 1/8 tsp each of salt and pepper, then soak sandwich in egg mixture.

Melt butter in a heavy medium skillet over medium-low heat. Cook sandwich, uncovered, until underside is well browned, about 4 minutes. Flip and cook remaining side, covered, until well browned, 3 to 4 minutes. Remove from heat and let stand, covered, 1 minute.