– 2 Tablespoons olive oil – ½ small onion , finely diced – 8 oz sliced fresh mushrooms – 3 cloves garlic , minced or pressed – 3 Tablespoons unsalted butter – Juice of 1 lemon – ⅓ cup reduced-sodium chicken broth – 1 teaspoon salt – Black pepper – Basil leaves , sliced For the Chicken: – 4 boneless skinless chicken breasts – ½ teaspoon salt – ½ teaspoon dried oregano – ¼ teaspoon black pepper – 4 thin slices Prosciutto (1 per chicken breast) – 4 thin slices Fontina cheese