Heat oven to 400°F. Lightly spread 1 side of each bread slice with about 1/4 teaspoon butter. Place half of bread slices onto aluminum foil-lined baking sheets, buttered-side down. Top each with about 1/2 teaspoon mustard, 1 tablespoon cheese and 1 bread slice, buttered-side up.
Bake, turning once, for 10 to 13 minutes or until golden brown and cheese is melted. Cut each sandwich in half. Serve immediately.
*Substitute honey mustard, orange marmalade, apple butter, cranberry relish, prepared basil pesto or herbed tomato paste.
Assemble sandwiches ahead on aluminum foil-lined baking sheets; cover with plastic food wrap. Refrigerate up to 4 hours. Bake as directed.