Cook the bacon. Place bacon in a cold skillet, turn heat to medium, and cook until crisp, flipping as needed. Drain on paper towels. Keep 1–2 teaspoons of bacon fat in the pan for extra flavor, or wipe clean and use butter for a milder finish.

Prep the apple. Core and thinly slice the apple. Aim for slices about 1/8 inch thick so they warm through and stay crisp, not mushy.

Butter the bread. Spread a thin, even layer of softened butter on one side of each bread slice. The buttered sides will face outward to help the bread toast evenly.

Assemble the first layer. Flip two slices so the unbuttered sides face up. Spread a light layer of mustard on each. Add a layer of cheddar, covering edge to edge. A little overlap is fine.

Add bacon and apple. Top the cheese with bacon strips, then shingle on the apple slices. Keep it even so the sandwich browns uniformly. Grind black pepper over the apples. If you like a touch of sweetness, drizzle a small amount of honey or swipe on apple butter.

Finish with cheese and bread. Add another light layer of cheddar on top of the apple. Cap each sandwich with the remaining bread slices, buttered sides facing out.

Grill low and slow. Heat a skillet or griddle over medium-low. Place the sandwiches in the pan. Cook 3–4 minutes per side, pressing gently with a spatula, until the bread is golden and the cheese is melted. Lower heat if the bread is browning too fast.

Rest and slice. Transfer to a cutting board and let sit for 1 minute so the cheese sets slightly. Slice in halves or thirds.

Serve. Pair with a green salad, tomato soup, or a handful of crisps. Enjoy immediately.