Preheat oven to 425.

In a small sauté pan add olive oil over medium heat and add onions. Sauté until golden and caramelized, about 10 minutes. Remove and set aside to cool.

With your knife, make a horizontal cut on the narrow side of the breast almost to the thick part of the breast and open. Salt and pepper the chicken breasts all over. Divide the onion mixture between all the chicken breasts and lay Fontina cheese over the onions and sprinkle half of the sage over the cheese. Use a toothpick to secure the chicken.

Wrap each chicken breast with a piece of prosciutto. At this point, the chicken breasts can be made the day before and refrigerated.

In a large oven safe skillet, add 1 tablespoon olive oil and over medium high heat brown each side of the chicken breasts. Place in oven and roast for 15-18 minutes or until the juices run clear when pierced with the tip of a knife. Remove from oven and place chicken on serving platter.

Set skillet on stove top over medium high and add wine and lemon juice to the pan drippings and scrape the bottom. Allow to come to a boil and add the remaining sage and cook for 2 minutes or until the sauce slightly thickens. Add 1 tablespoon of butter (optional) and swirl pan to emulsify the sauce and allow it to melt. Pour the sauce over chicken breasts and serve.