Whisk 1/4 cup red wine vinegar, 2 minced shallots, 1 1/2 tsp. ground coriander, 3/4 tsp. kosher salt and 1/4 tsp. black pepper in a medium bowl.
Whisk in 1/2 cup olive oil. Stir in 3 (10 oz.) packages of small white mushrooms, thinly sliced, and 4 thinly sliced scallions. Serves 8.