Place the red wine vinegar, apple cider vinegar, the mustards, and honey in a food processor. Turn on and drizzle in the olive oil until fully incorporated (emulsified).
½ tablespoon red wine vinegar, 1 ½ tablespoons apple cider vinegar, ½ teaspoon Dijon mustard, ¼ teaspoon dry mustard, 1 teaspoon honey, 3 ½ tablespoons olive oil
Marinate the chicken breasts overnight in this mixture (I did 30 minutes after reading an article in Cook's Illustrated that 30 minutes is all that's needed for marinating chicken).
2 large chicken breasts
Make the Stuffing:
Peel, core, and very roughly chop the apple. Melt the butter in a saute pan over medium/high heat and add the apples. Cook for roughly 5 to 10 minutes or until deeply browned. Remove from heat and transfer to a mixing bowl. Once cooled, add the fontina and cherries and mix. Season to taste with salt and pepper.
Salt and freshly ground black pepper, ½ tablespoon unsalted butter, 1 medium Fuji apple, 1 ½ ounces fontina cheese, 1 ½ ounces dried cherries
Assemble and Bake the Chicken:
Preheat grill or grill pan on high. Remove chicken breasts from the marinade and transfer to a plate. Season with salt and pepper. Mark the chicken breasts by grilling them only long enough to make grill marks, then rotate them about 45 degrees to make crosshatch marks. Do this on 1 side only. The chicken breasts will still be completely raw inside.
Preheat the oven to 400°F. Cut a slit in the side of each breast (in the thicker part) about 1-inch wide, creating a pocket almost as large as the breast itself. Stuff the breasts with the apple stuffing mix - cramming in as much as possible without ripping the breast. At this point the breasts are ready to be baked or can be held in the refrigerator for up to 24 hours
Place chicken on a baking sheet and bake for about 12-15 minutes or until an instant-read thermometer registers 165°F in the thickest part of the breast. (Mine took longer to cook. I don't have an instant-read thermometer so I had to cut and check for pink.)
Notes
Source: Adapted from FoodNetwork.comNutritional information is only an estimate and will vary based on your ingredients and substitutions. You're Gonna Bake It After Allbakeitafterall.com