1Preheat oven to 375 degrees. Place rack in middle.
Heat 10 inch cast iron skillet over medium high heat. Add 1 tablespoon oil to pan. When oil shimmers add asparagus and 1/2 teaspoon salt, and cook tossing occasionally until asparagus is crisp and tender and brown in spots 3-4 minutes. Squeeze lemon juice over asparagus and transfer to plate to cool. Reserve skillet.
2Heat 1 tablespoon of oil in skillet over medium high heat. When oil shimmers add onions and cook stirring occasionally until translucent about 3 minutes. Add garlic, spinach, pepper and 1 teaspoon of salt and cook 1 minute until spinach is warmed through.
3Whisk eggs and milk in medium bowl until thoroughly combined. Pour into skillet and cook stirring constantly just until eggs begin to scramble but are still very wet. Remove from heat.
Distribute asparagus in a single layer over eggs pressing them gently into eggs.
Transfer to oven and bake just until frittata is set, 18-20 minutes. Cool 5 minutes before slicing and serving.
Combine capers, scallions. parsley, remaining 1 Tablespoon oil, vinegar, and remaining 1/4 teaspoon salt in small bowl. Serve over frittata.