1. Move one oven rack to the top position (in preparation for broiling) and preheat the oven to 425° F. Grease a 10 to 12-inch oven-safe skillet with olive oil.
2. Lay the sliced chicken breast open and lightly season both sides with salt and pepper. (Note: Most 1.5-pound packages of Mary’s Chicken contain three breasts. The following instructions are written for three chicken breasts. If there are only two in the package, adjust the instructions as needed.) Place two slices of cheese on the inside of each of the three breasts and fold the chicken breast over. Wrap the prosciutto around the breasts (two pieces around one breast and 1 ½ pieces around the other two). Transfer the chicken to the prepared skillet. Scatter the garlic, onion, and apple slices around the chicken. Drizzle the olive oil over the garlic, onion, and apples and transfer to the oven. Roast for 15 minutes (if roasting two larger breasts, add 3 minutes to the cooking time). After 15 minutes, move the chicken to the top rack and broil for 3 minutes, until the prosciutto begins to darken (watch closely to prevent the prosciutto from burning). Remove from the oven and transfer the chicken to a plate.
3. While the chicken is in the oven, prepare the sage butter: Melt the butter in a small skillet over medium heat. When the butter begins to foam, add the sage leaves (make sure they are dry), stir to spread them out, and sauté until the sage leaves become crisp. Stir once or twice to ensure even cooking, but do not over stir. Remove from heat and set aside.
4. Place the skillet with the garlic, onion, and apples over medium-high heat. Add the apple juice and vinegar. Increase the heat to high and bring to a boil. Boil for 5 minutes, until reduced by half, and season to taste with salt and pepper. Reduce the heat to low, and when the sauce returns to a gentle simmer, slide the chicken back into the skillet. Simmer for 1 minute, spooning the sauce over the chicken. Remove from the heat.
5. Transfer the chicken to a cutting board and slice into ½-inch slices. Evenly divide the onion-apple mixture among four plates and top each with the sliced chicken. Drizzle with the pan sauce and top with the fried sage and brown butter. Serve with rice, cauliflower rice, roasted vegetables, baked potatoes, or salad.