Blend flour, salt, oregano, and pepper. Set aside 1 tablespoon of the flour mix; coat chicken with remaining mixture. In heavy skillet, brown chicken in butter. Combine wine and lemon juice; pour over chicken. Add onion and bring to boiling; reduce heat. Cover and cook over low heat for 45 to 60 minutes until chicken is tender. Meanwhile, in small skillet, cook mushrooms in butter until tender. Drain. Remove chicken to platter; keep warm. Add mushrooms, tomatoes, and sugar to wine mixture in skillet. Cook until vegetables are tender, about 5 minutes. Blend together the reserved flour mixture and the water. Add to sauce. Cook and stir until thickened and bubbly. Pour over chicken on serving platter.