For the mustard sauce
Prepare ahead
1. Snap off the tough ends of the asparagus. Peel the lower half of each asparagus with a vegetable peeler. Fill a large frying pan with enough water to come ½ in (1½ cm) up the sides and bring to a simmer. Add the asparagus. Cover, and cook 3–4 minutes, or until just tender. Drain and rinse under cold running water. Pat dry with paper towels. Refrigerate while making the sauce.
2. For the sauce, whisk the oil, vinegar, mustard, and tarragon together in a bowl. Whisk in the yogurt until smooth. Season with salt and pepper.
3. Divide the asparagus among salad plates. Top with the mustard sauce and sprinkle with the red onion. Serve immediately.
Variation
Asparagus with Creamy Garlic Sauce
Combine ½ cup plain yogurt, 1 tbsp chopped chives,1 garlic clove, ½ tsp finely grated lemon zest, and a pinch of cayenne pepper in a bowl. Chill, covered, for at least 2 hours. Remove the garlic before serving.
