Preheat oven to 375° F.

Melt butter in a 10-inch ovenproof sauté pan over medium heat.

Cook onions and peppers in butter for 5 minutes, stirring occasionally.

In a large bowl, beat eggs with salt, pepper, basil and spinach if using.

Pour egg mixture into the pan. Cook until edges start to set, about 3 minutes. Sprinkle feta over eggs.

Transfer pan to the oven and bake for 20 minutes or until puffy and golden. Using a pot holder, shake the pan to make sure egg mixture is set and no longer moving. Frittata will puff up and be slightly brown on top when it is done.

Cool frittata in the pan on a wire rack for about 10 minutes. Place a plate over the top of the pan and invert the frittata onto the plate. Cut into wedges.