In a large non-stick skillet, heat 2 tablespoons of olive oil over medium high heat. Add the onions and saute for 2-3 minutes (until beginning to soften). Add the mushrooms and continue to cook until mushrooms have softened and begin to golden (5-10 minutes). Add the garlic and saute for another minute, until fragrant.
Add butter and lemon juice and lemon zest, if using; stir well.
Add chicken broth salt and pepper. Allow sauce to simmer and thicken a bit (5-10 minutes). Keep warm until ready to use. Add chopped basil to sauce just before serving.
For the Chicken:
Mix together the salt, oregano, and pepper and sprinkle mixture even over both sides of the chicken breasts. Grill over medium high heat until cooking through (8-10 minutes per side).
Place cooked chicken breasts on a sheet pan and add one slice of prosciutto to each chicken breast, then top with 1 ounce of fontina cheese. Broil until cheese in melted. Place chicken breasts on serving splatter and spoon the lemon mushroom sauce evenly over top of them.