1Heat oven to 325°F. In small bowl, mix mustard, butter, thyme, marjoram, 1/2 teaspoon salt and 1/4 teaspoon pepper.
2In shallow roasting pan, place turkey breast, skin side up. Brush with mustard mixture. Place 1/2 cup of the broth in bottom of pan; refrigerate remaining broth. Insert ovenproof meat thermometer so tip is in thickest part of turkey breast and does not touch bone. Spray piece of foil with cooking spray; cover turkey. (Foil does not need to tightly cover turkey; secure foil to each end of pan.)
3Bake 1 hour. Add mushrooms and onions to pan; spoon pan drippings over top. Bake uncovered 1 hour to 1 hour 30 minutes longer or until thermometer reads 170°F. Place turkey on platter; cover with foil to keep warm. Let stand 15 minutes for easiest carving.
4Pour pan drippings (without vegetables) into measuring cup. Add remaining broth to drippings to equal 1 1/2 cups; pour into 2-quart saucepan. In small bowl, mix flour and cold water until smooth; stir into drippings in saucepan. Heat to boiling over medium-high heat, stirring constantly with wire whisk. Boil and stir about 1 minute or until mixture thickens. Stir in mushrooms and onions from pan. Simmer 5 minutes, stirring occasionally. Stir in salt and pepper to taste. Serve gravy with turkey.