- 1/3 cup red wine vinegar
- 2 teaspoons sugar
- 1 small red onion, halved through core and thinly sliced crosswise
- 1/2 cup whole-grain mustard
- 1/4 cup honey
- 8 slices pepper jack cheese
- 1/2 pound thinly sliced pastrami
- 1 1/2 pounds ground chicken (a mix of white and dark meat)
- 4 potato sandwich rolls or hamburger buns, toasted
In a bowl, whisk together the vinegar, sugar, 2/3 cup water and 1 teaspoon salt until dissolved. Pack the sliced onion into a 1-pint jar; top with vinegar mixture. Let sit at room temperature about 1 hour.
Measure out 1/2 cup of the drained, pickled onions; chop. (Refrigerate remaining onions for up to two weeks.) Transfer them to a bowl then whisk with the mustard and honey; season relish with salt and pepper.
Heat a griddle pan, a large, heavy skillet or a grill over medium-high. Arrange 4 slices of cheese on a work surface. Pile one-quarter of the pastrami on top of each slice of cheese. Top the pastrami piles with the remaining slices of cheese.
Form the ground chicken into 4 patties (thinner in the centers for even cooking); season. Cook until cooked through, about 6 minutes per side. After flipping, top with pastrami and cheese; tent with foil to melt the cheese.
Spread some relish on the roll bottoms, then add the patties and roll tops. Serve with the remaining relish.