- Adjust oven rack to middle position and heat oven to 425 degrees. Microwave vinegar, ¼ cup honey, and ½ teaspoon salt in bowl until bubbling, about 3 minutes. Stir in apple and let stand for 5 minutes. Drain apple (brine can be saved and reused up to 2 more times).
- Whisk ¼ cup mayonnaise, mustard, rosemary, remaining 1 tablespoon honey, and remaining ¼ teaspoon salt together in bowl. Spread 1 side of each slice of bread with remaining ¼ cup mayonnaise. Flip bread and distribute mayonnaise-mustard mixture among slices. Distribute 4 slices of cheddar among 4 slices of bread, followed by turkey; apple; remaining 4 slices of cheddar; and remaining 4 slices of bread, mayonnaise side up.
- Transfer sandwiches to rimmed baking sheet and bake until cheddar is melted, about 15 minutes, flipping sandwiches halfway through baking. Serve.
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