Heat oven to 200°F.
Add olive oil to 12-inch skillet, and heat over medium high heat. Season chicken breasts with salt and pepper. Dredge chicken breasts in flour. Place chicken breasts in skillet and lightly fry until golden brown on both sides, about 3 minutes on each side. Remove chicken from skillet and place on rimmed baking sheet in heated oven to keep warm.
Wipe skillet clean. Add pancetta, or bacon pieces, to skillet and cook until crispy. When cooked through place pancetta or bacon on chicken in the oven. Add mushrooms to oil and sauté until softened. Add garlic and tomato paste. Remove skillet from heat and add Marsala. Return pan to heat and add lemon juice and half stick of butter. Bring mixture to simmer and allow it to thicken. Once it has thickened, pour sauce over chicken on rimmed baking sheet in oven to keep warm.
Start pot of heavily salted water for pasta and cook according to directions. While pasta is cooking, melt butter over medium heat in skillet that was used for chicken marsala. Sprinkle in flour and whisk to create paste. Slowly whisk in milk to make béchamel. Sprinkle in fontina and cheddar cheese and whisk until melted.
Top pasta with chicken marsala and garnish with minced parsley leaves.