Egg Salad Sandwiches with Watercress

10 large hard-boiled egg whites
3 Tbsp. non-fat sour cream or non-fat plain yogurt
1 Tbsp. fat-free mayonnaise
1 Tbsp. grainy mustard
4 scallions, trimmed and chopped
Freshly ground black pepper to taste
¾ cup washed and stemmed watercress
6 slices 100% whole wheat bread

Chop egg whites and stir in sour cream (or yogurt), mayonnaise and mustard.  Add scallions and season with pepper.

Arrange watercress on 4 bread slices.  Top with the egg salad and cover with the remaining bread slices.