What makes this classic dish special is the rich sauce that comes together with sweet, tart Marsala cooking wine after chicken and mushrooms are seared to golden brown. We add wilted spinach for flavor and color and serve it over pasta to create a homey Italian classic.
Bring a saucepan of water to boil. Salt generously and add pasta. Cook until al dente, tender but not soggy.
Heat a skillet with oil over medium-high heat. To heated oil, add chicken and sear on both sides until golden brown and cooked through, ~3 minutes on each side (do this in batches and add additional oil if your pan cannot hold all of the chicken). Transfer chicken to a plate and return skillet to the heat.
Reduce heat to medium and add butter. When the butter melts, add mushrooms and saute, scraping up any bits on the bottom of the pan, until mushrooms are golden brown, 5 to 6 minutes. Add shallots and saute for 1 minute to soften.
Sprinkle flour over mushrooms and stir until no dry spots remain. Slowly pour wine and stock over mushrooms while whisking. Bring to a simmer. When the liquids are simmering, stir in spinach and cook until spinach is wilted and sauce is reduced slightly. Season with salt and pepper.
Return chicken to the pan and spoon sauce over chicken. Squeeze lemon juice over chicken and top with parsley.