- 1 onion, thinly sliced
- 1/3 cup red wine vinegar
- 1/3 cup vegetable oil
- 1 tablespoon brown sugar
- 1 teaspoon Dijon mustard
- 2 (12 ounce) cans whole mushrooms, drained
- 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.