My mother used to call this "Mock-Hollandaise." It's a simple dip that got my youngest son interested in eating raw broccoli. Slip in a few sweet onion rings too, just to see if the kids will like them.

1/2 cup mayonnaise (or half mayonnaise & half plain, low-fat yogurt)
1-2 tablespoons mustard
1 to 2 teaspoons red or white wine vinegar