Gather all ingredients before starting the recipe.
Spread 1/2 cup all-purpose flour onto a sheet of parchment paper.
Remove bone from 4 chicken thighs and place between 2 sheets of cling wrap. Gently pound the chicken thighs, skin side down, to 1/2" to 1/4" thickness. Season with kosher salt and black pepper.
Divide 4 slices prosciutto, one slice per piece of chicken and top the prosciutto with fresh baby spinach.
Fold each chicken thigh in half like a book, then dredge each piece in flour.
Add 4 tbsp olive oil in a large skillet and heat over medium high heat.
Add chicken and sauté until deeply golden brown, both sides, between 5-6 minutes per side. Remove chicken to a plate and reserve. Drain all but 1 tbsp of oil from pan.
Reduce heat to medium and add 5-6 cloves sliced garlic, 4 medium sliced shallots and 1/2 lb sliced mushrooms to the pan and sauté until the mushrooms have sweated, about 3-4 minutes.
Add 3/4 cup sweet marsala wine and turn up heat to medium high and simmer until the wine is reduced by a quarter, 3-4 minutes.
Add 1/2 cup chicken stock and cook for another 3-4 minutes. Reduce heat to low and swirl in 2 tbsp unsalted butter and stir until fully incorporated.
Garnish with 1/4 cup parsley chopped and serve.