Steamed Fiddleheads with Leek Greens

1 pound fiddleheads, cleaned
1/2 cup plain yogurt
1/2 cup mayonnaise
1 tablespoon fresh lemon juice, or to taste
3 teaspoons Dijon mustard
3 tablespoons finely chopped leek greens

Steam the fiddleheads over boiling water for 5 minutes, or until they are crisp-tender.

Drain, then chill in a bowl of cold water and ice to stop the cooking. When they are cool, transfer to a colander to drain.

In a small bowl, whisk together yogurt, mayonnaise, lemon juice, mustard and leek greens. Add salt and pepper to taste, whisking until the sauce is smooth. Serve the fiddleheads topped with the sauce.

Serves 4 to 6.