Grate fontina cheese. Clean and slice oyster mushrooms into ½" strips. Preheat oven to 375f and set rack to middle.
In an oven safe pan heat 1-2 Tbsp butter and 1 Tbsp olive oil (enough to coat pan) on medium heat. Pat dry chicken cutlets and lightly season both sides with salt and pepper. Dredge cutlets in flour and shake off excess. Pan sear the cutlets for 2 minutes per side then remove cutlets and set aside. If required work in batches adding another Tbsp of butter and olive oil to second batch.
In the same pan saute mushrooms over medium-high heat for 5-7 minutes or until brown. Use a touch more oil and butter if the pan is dry. When mushrooms are finished remove and set aside.
Turn heat to high and add the ¾ cup chicken stock and ¾ cup white wine. Cook for 5 minutes to let the sauce reduce. Scrape the bottom of the pan with a wooden spoon to dislodge all brown bits. Add in 1 Tbsp of butter to the sauce and stir to combine.
Turn heat off and place the chicken in the pan. Top chicken with 1 slice of prosciutto per cutlet. Next, place the mushrooms on top and finally evenly distribute the fontina cheese. Spoon a touch of the sauce on top of each cutlet and place pan in oven to finish cooking.
Remove the chicken valdostana when the cheese is bubbly and the chicken is fully cooked through (about 5-7 minutes). Enjoy!
Video
Notes
Fontina cheese, prosciutto and chicken stock contain salt. Adjust extra salt accordingly.
Mozzarella or soft provolone can be substituted for the fontina.