Cook asparagus in a large saucepan of boiling water 2 minutes.
Drain and plunge asparagus into ice water; drain and pat dry.
Combine milk and next 4 ingredients (through egg whites) in a medium bowl, stirring with a whisk. Stir in asparagus, 2 tablespoons cheese, dill, and trout.
Heat a 10-inch ovenproof skillet over medium heat. Coat pan with cooking spray.
Add oil and onions to pan; cook 1 minute, stirring occasionally.
Pour egg mixture into pan; stir once. Cook without stirring for 2 minutes or until edges begin to set.
Place pan in oven.
Bake at 450 for 8 minutes or until eggs are just set.
Remove from oven; sprinkle evenly with remaining 2 tablespoons cheese.
Preheat broiler.
Broil frittata 2 minutes or until lightly browned.