Bring onion, vinegar, oil, sugar, and Dijon mustard to a boil in a saucepan. Add mushrooms and simmer until liquid is slightly reduced, 5 to 6 minutes. Transfer mixture to a covered container and chill. Drain before serving.
Tips & Tricks
How to Make Stuffed Mushrooms
Enjoy baked mushroom caps stuffed with a delicious vegetarian filling.
Salisbury Steak with Mushrooms
Watch how to make a classic Salisbury steak dinner.