With this technique, you'll have a perfect crispy-melty sandwich without a panini press. A number of different cheeses work equally well here, so feel free to experiment.
1-1/2 tablespoons salted butter, at room temperature
4 slices sourdough bread
2 teaspoons whole-grain mustard
1/3 large firm-tart apple (about 3 ounces; see "Apple notes," below), unpeeled cored, and sliced very thin
4 ounces sharp Cheddar, aged Gouda, Gruy
You'll need one large skillet and one medium-size one, preferably cast-iron (the heavier the better). Set both skillets over medium heat and let them get hot.
Meanwhile, butter one side of each bread slice; then lay slices butter side down on your cutting board. Spread equal portions of mustard on two slices; then divide apple slices into two portions and lay on top of mustard. Divide cheese slices and lay over apples. Top sandwiches with remaining bread slices, buttered side up.
Lay sandwiches in the large skillet. Cook until bottoms are browned, 3-4 minutes; then flip. Set the preheated medium-size skillet on top of sandwiches as a press. (If your skillet isn't heavy, weigh it down with a water-filled kettle or a few large cans of tomatoes.) Cook until both sides are evenly browned, about 2 minutes more.
Apple notes: Use whatever variety you have on hand, but a firm-tart apple goes well with the mustard and cheese in the sandwich.