Heat a large nonstick skillet over medium heat. Preheat oven to 400 degrees F.
Place cutlets on a plastic cutting board and season with salt and pepper. Top with chopped sage, 2 slices prosciutto each and a few thin slices of fontina. Roll tightly and secure with toothpicks. Season outside of rolls with more salt and pepper, then dredge in flour.
Add 2 tablespoons extra-virgin olive oil, 2 turns of the pan and 1 tablespoon butter to the pan. Add chicken and raise heat to medium high. Brown the rolls evenly, 5 minutes, then place on a nonstick cookie sheet and transfer to oven to roast for 15 minutes longer. Add another tablespoon butter to the pan and melt it. Add mushrooms to the pan and cook, stirring frequently, until just tender 3 or 4 minutes. Add flour and cook another minute. Whisk in stock. Season pan gravy with mushrooms with salt and pepper and turn off heat. Reheat with moderate flame when you remove chicken rolls from the oven and you are ready to serve.
Preheat a second skillet over medium heat. Add extra-virgin olive oil 2 turns of the pan, and the garlic. Add the spinach to the pan in bunches and turn to wilt. Add more spinach as it wilts down until all the spinach is in the pan. Season with salt and pepper, to taste and serve warm.
To plate, leave rolls whole and spill sauce over top, spinach alongside, or, slice rolls and fan them out over a bed of spinach and top with mushroom sauce.
*Ask butcher to butterfly the breasts into large cutlets for you. This is a real time-saver! If a butcher is not available to you, cut into each breast from the fatter side and work across the breast, leaving them attached at the opposite side. Open the breast up and pound lightly with a mallet to about a 1/4-inch thickness. Also, in many supermarkets chicken cutlets are available in packages. They are small pieces of thinly sliced breast meat. Simply adapt the method and make 2 small roll-ups rather than 1 large roll up per portion.