Preheat oven to 350 °
Over medium-low heat, melt butter or coconut oil in an oven proof skillet; be sure to coat the sides of the pan, until pan is hot but not smoking
Beat eggs well, adding salt and pepper and pour into hot pan
Cook for a few minutes until bottom and sides are set
Sprinkle with chopped greens and peppers
Place pan in oven and cook for 7-10 minutes, until eggs are set
Carefully remove the hot pan from the oven and turn frittata out on cutting board
Cut into wedges and serve