FOR MARINADE: Combine brown sugar, lemon juice, Worcestershire sauce, salt, and herbs. Stir in buttermilk. Add meat; cover and marinate in refrigerator a minimum of 12 hours. FOR KABOBS: Parboil onions in boiling salted water to cover, 6-8 minutes; rinse with cold water and drain. TO ASSEMBLE: Alternate pieces of meat with onions, zucchini, tomatoes, and mushrooms on each skewer. Grill over hot coals until meat reaches desired degree of doneness. Baste frequently with marinade. Allow approximately 15 minutes for medium cooked meat. Makes 8 kabobs. From Massachusetts Dairy Sampler by Massachusetts Dairy Industry