(Lavon from bridge club)

In a small saucepan combine 1/3 cup red wine vinegar, 1/3 cup salad oil, 1 small onion, thinly sliced and in rings, 1 tsp. salt, 2 Tbsp. dried parsley, 1 tsp. mustard, and 1 Tbsp. brown sugar.  Bring to boil.  Add 1 lb. fresh mushrooms.  Simmer 5-6 minutes.
Chill in covered bowl several hours, stirring occasionally.