1 lb small to medium fresh white button mushrooms
1 small onion, sliced thin
1/3 cup white wine vinegar
1/3 cup canola or extra virgin olive oil
3/4 tsp salt
dash freshly ground black pepper
1 tsp ground mustard
Combine all the ingredients together in a large saucepan; bring to a boil over medium heat. Cook for 6 minutes, stirring only once. Allow to cool to room temperature then transfer to a bowl with a lid. Cover dish and refrigerate overnight.
Garnish with fresh parsley or cilantro if desired.