Blanch the mushroom slices in boiling water for 2 minutes. Drain and set aside. Rinse the chicken under cold water and pat dry with paper towels. Dredge the chicken in flour, shaking off the excess. Heat the olive oil in a large skillet and sauté the chicken breast halves until lightly browned, about 2 minutes on each side. Add the diced onions and sauté until lightly browned. Drain excess oil from skillet and add the tomato sauce, white wine, Marsala, stock, and butter. Bring to a boil. Reduce heat, cover, and simmer for 10 minutes. Season with salt and pepper, to taste.