Combine the first 4 ingredients in a bowl. Spread 1 teaspoon butter on one side of each bread slice. Spread 2 tablespoons mustard mixture over unbuttered side of each of 2 bread slices; top each with 3 turkey slices, 2 tomato slices, 1 cheese slice, and 1 bread slice (with buttered side out).
Heat a large nonstick skillet over medium heat until hot. Add the sandwiches; cover and cook for 3 minutes on each side or until golden brown.