To blanch the asparagus:
Wash and trim asparagus.
Bring a large pot of salted water to a boil (about 1 1/2 Tbsp. salt per every 5 qt. of water). Boil asparagus until they are bright green and crisp-tender (1 to 2 minutes).
Transfer asparagus to ice water to set the color and texture, using a slotted spoon or tongs. Chill until ice-cold.
Drain asparagus and pat dry.
Stir together cream cheese, lemon juice, grated onion, and 1/4 teaspoon salt until well blended.
Spread 1 Tbsp. of the cream cheese mixture on each bread slice.
Cut the blanched asparagus into thin, ribbon-like strips, using a vegetable peeler.
Divide asparagus and prosciutto among prepared bread slices. Gently press to adhere.
Cover with plastic wrap, and chill 15 minutes to 12 hours.
Trim crusts from chilled slices, and cut each slice into 2 triangles.
Serve with other varieties such as Vegetable Medley Tea Sandwiches to create a mosaic look.