Preheat oven to 375 degrees. Spread half the bread slices with mustard. Layer with cheddar, ham, apple, and more cheddar, dividing evenly. Top with remaining bread slices. Lightly butter both sides of sandwiches.
Heat a large nonstick skillet over medium heat. Working in batches, cook sandwiches, turning once, until bread is golden, about 4 minutes per side. Transfer sandwiches to a rimmed baking sheet. Bake in oven until cheddar is melted, about 5 minutes. Cut each sandwich in half on the diagonal, and serve warm.