Combine first 7 ingredients and form into small patties (at least 12 patties). Brown patties on both sides in skillet. Place patties on bottom of Crock-Pot. Slice onion (cut in half lengthwise, turn flat edge down, and cut into half circles). Add layer of onions and mushrooms. Mix beef broth, gravy mix, ketchup and mustard and pour over. Put lid on Crock-Pot. Cook on low for 5 hours. Remove patties and set aside. Turn Crock-Pot to high. Whisk in cornstarch until thickened. Return Salisbury patties to Crock-Pot to coat patties with the sauce. Can serve immediately or turn to low until ready to serve. Yields 12 to 14 servings.