Asparagus, Basil and Feta Fritatta

10 eggs
¼ cup feta cheese, crumbled
3 tablespoons extra-virgin olive oil
8 large leaves fresh basil, cut into ribbons
¼ cup fresh parsley, chopped
1 pound fresh asparagus, cut into 2-inch sections and steamed until tender
2 tablespoons butter

Whisk eggs until fluffy. Whisk in other ingredients except butter. Heat butter in 10-inch oven-safe cast iron pan until medium hot. Tilt pan around so butter coats all interior surfaces. Pour egg mixture into pan, cover pan, and reduce heat. Cook for about 8 minutes. Slide frittata out of pan onto plate, then put another plate on top of it and invert. Slide the frittata, uncooked side down, into pan and cook for another 4 minutes.
Remove from heat and cut into wedges.