2 tablespoons unsalted butter, at room temperature
2 pieces pieces sourdough bread, cut into ½-inch thick slices
2 ½ ounces Brie, slightly chilled
6 thin slices Granny Smith apple
1 large handful arugula, about 1 cup
2 teaspoons Colman’s Prepared Mustard, plus additional for serving
Slather one side of each slice of bread with ½ tablespoon butter. Place the bread slices, buttered side down, on a work surface.
Spread the Colman's mustard on one slice. On the opposite slice, lay the apple slices down so that they overlap slightly and cover the whole slice.
Remove the rind from the Brie. Lay the Brie on top of the apple, spreading and flattening as you go. Add arugula and press to adhere. Close the sandwich so that both buttered sides are on the outside.
Heat a skillet over medium heat. Add 1/2 tablespoon butter and swirl to coat. When the butter is sizzling, add the sandwich, and using a spatula (I like to use a fish spatula) or your hand, press down to begin to toast. Cook until golden and crispy, about 2-3 minutes. Add remaining butter to the skillet and flip the sandwich. Continue cooking, lightly pressing again with the spatula, until bread is golden and cheese is gooey, about 2 minutes more.
Transfer to a cutting board and cut in half. Serve immediately, with additional mustard on the side for dipping. (To go full on cheese, mind the pun: cut the sandwich on the diagonal, and flip one side over to form a heart.)