Roast-Beef Salad with Creamy Horseradish Dressing

INGREDIENTS

1/3 cup

mayonnaise

1/3 cup

sour cream

3 tbsp

drained bottled horseradish

1 tsp

red- or white-wine vinegar

1/4 tsp

fresh-ground black pepper

3/4 tsp

salt

large head escarole (about 1 1/2 pounds), torn into bite-size pieces (about 4 quarts)

1/4 lb

mushrooms, sliced thin

small red onion, sliced thin

1/2 lb

sliced roast beef, cut into approximately 1 1/2-by-1/2-inch strips