Pound each chicken breast in between two pieces of wax paper to 1/4 inch thickness. Sprinkle both sides with salt and paper. Brush each piece of chicken with lightly beaten egg. Wrap each piece with two pieces of prosciutto. Place in refrigerator for 15 minutes.
Add olive oil to large saute pan and place over medium-high heat. When the pan is hot add the chicken and saute for 3 minutes each side until golden brown and cooked through. Transfer to plate and cover with foil.
To the same pan add the butter. Reduce the heat to medium and saute the shallots for 5 minutes. Add flour and cook for 2 minutes, stirring constantly. Add the thyme leaves and wine. Simmer for 3 minutes scraping up the brown bits from the bottom of the pan. Add the chicken stock and the chopped sage and continue to simmer for a few minutes until thickened. Place the chicken on top of the sauce in the pan and top each piece of chicken with the slice of fontina and a whole sage leaf. Place lid on saute pan until cheese is melted. Serve the chicken over a few spoonfuls of the sauce.
This recipe was adapted from Ted Allen's recipe for Chicken Paillards with Pancetta and Sage.