Place mushrooms, butter and onion in an 20 cm square glass dish. Cover with plastic wrap. Stirring midway through cooking, microwave on high 2 minutes. Arrange chicken on top of mushroom mixture. Add wine, marjoram, salt and pepper. Re-cover and microwave on high 5-6 minutes, or until chicken is no longer pink. Combine cream, mustard and flour in a 2-cup glass measure. Drain liquid from chicken into it; re-cover chicken and set aside. Stirring after each minute, microwave on high 3-4 minutes, or until mixture thickens and bubbles. Stir grapes into sauce and microwave on high 1-2 minutes, or until grapes are heated through. Slice chicken crosswise and serve on warm plates topped with sauce.