Open-Face Chicken Salad Sandwiches

INGREDIENTS

• 1 pound slender asparagus, trimmed
• 1/2 cup mayonnaise
• 1/4 cup plain nonfat Greek yogurt
• 1 tablespoon fresh lemon juice
• 1 tablespoon Dijon mustard
• 4 cups skinless cooked chicken (from 1 rotisserie chicken), cut into bite-size pieces
• 1/2 cup chopped fresh chives
• 6 thin prosciutto slices
• 6 1/2-inch thick slices rye bread, lightly toasted
• 2 green onions, thinly sliced