Tarragon-Mustard Sauce

½ cup firmly packed mayonnaise (use “light,” if you like)
½ cup firmly packed low-fat plain yogurt
1 tablespoon Dijon mustard
1 medium scallion, trimmed and finely chopped (include some green tops)
2 tablespoons coarsely chopped parsley
1 tablespoon coarsely chopped fresh tarragon or 1 teaspoon crumbled dried leaf tarragon
½ teaspoon finely grated lemon zest
½ teaspoon tomato ketchup, or to taste
½ teaspoon hot red pepper sauce, or to taste

This sauce will taste even better if you make it several hours ahead of time and refrigerate until ready to serve.

Whisk the mayonnaise, yogurt, and mustard together until smooth and creamy, then mix in all remaining ingredients. Taste for ketchup and hot red pepper sauce, and adjust as needed. The ketchup is to mellow the tartness of the yogurt, not to add sweetness.
Cover the sauce and refrigerate for several hours or even overnight.
Let the sauce stand at room temperature for about 30 minutes before serving.