Winter White Salad

Name: Winter White Salad
Ingredients: 1 tablespoon whole-grain or coarse-grain mustard; 2 tablespoons white-wine vinegar; 1/2 cup heavy cream; 1/2 teaspoon salt; 1/4 teaspoon white pepper; 1/2 medium head cauliflower (3/4 pound), stems discarded and florets cut lengthwise into 1/4-inch slices (3 cups); 2 Belgian endives, leaves separated and cut diagonally into 1/2-inch pieces; 1 small head Napa cabbage (1/2 pound), greenish outer leaves discarded and pale inner leaves cut crosswise into 1-inch pieces (5 cups loosely packed); 1 small onion, finely chopped and soaked in cold water 10 minutes, then drained