Place flour, salt and pepper in a pint-size food storage bag, and shake chicken to coat. In a large, nonstick skillet, melt the butter with 1 Tbsp. of oil and lightly brown the chicken pieces. Remove chicken pieces from skillet. Add additional olive oil to the skillet and add onions; cook for about 5 minutes or until tender. Stir in green pepper and garlic cooking for about 3 minutes. Stir in sherry and bring to a light simmer. Add diced tomatoes, crushed tomatoes, mushrooms and oregano. Lightly salt & pepper while bringing to a simmer. Return chicken to skillet.

Cover and simmer on low for about 20 minutes or until chicken is cooked through.