1. Preheat oven to 450°.
2. Cook asparagus in a large saucepan of boiling water 2 minutes. Drain and plunge asparagus into ice water; drain and pat dry.
3. 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.
4. 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.
5. Preheat broiler.
6. Broil frittata 2 minutes or until lightly browned. Remove from oven; let stand 5 minutes. Cut into 8 wedges.