Steamed fiddleheads with horseradish scallion sauce

Ingredients

• 1 pound fiddleheads, cleaned
• 1/2 cup plain yogurt
• 1/2 cup mayonnaise
• 1 tbsp fresh lemon juice, or to taste
• 2 tsp Dijon-style mustard
• 1 tbsp drained bottled horseradish, or to taste
• 3 tbsp finely chopped scallion green finely chopped scallion green finely chopped scallion green