1 bunch watercress, tough stems removed, coarsely chopped or torn (about 2 cups)
3 Tbsp parsley, fresh, leaves
1 medium scallion(s), chopped (about 2 Tbsp)
3 Tbsp basil, fresh, leaves
1/2 cup plain fat-free yogurt, Greek-style
1/2 tsp table salt

In bowl of a blender or food processor, place all ingredients and pulse until well combined. Pour into a bowl or plastic container and refrigerate until ready to use.  Yields about 1/4 cup of sauce per serving.  This sauce will keep for up to 3 days, covered, in the refrigerator.

You can substitute any fresh herb for either the parsley or the basil. And consider swapping spinach for the watercress for a milder flavor. Or for a peppery flavor, use arugula instead of the watercress.