1/4 c. (1/2 stick) unsalted butter, room temperature
2 tablespoons Dijon mustard
4 large slices of sourdough bread, halved
1 Pink Lady apple, cored and thinly sliced
2 cups Gouda cheese, shredded
In a small bowl, mix the softened butter and mustard together until smooth.
Preheat oven to 350° F. To assemble tartines, place bread slices on a parchment-lined baking sheet. Smear each piece of bread with mustard/butter mixture. Divide sliced apple among the bread and top liberally with cheese.
Transfer to the oven to bake for 7–10 minutes, until cheese is melted. Turn oven to "broiler" and transfer the tartines under broiler for an additional minute, just until cheese is golden brown and bubbly. Sprinkle each tartine with a bit of black pepper and serve.