Step 1

Preheat the oven to 375°F.

Step 2

In a heavy skillet, add olive oil and bring to medium-low heat.

Step 3

Saute onions and peppers until vegetables are tender, about 7 minutes.

Step 4

Sprinkle the mixture with salt and pepper.

Step 5

Pour egg whites into the skillet and cook for 3 minutes.

Step 6

Sprinkle the top with feta and spinach.

Step 7

Put skillet in oven and bake, uncovered, for 8 to 10 minutes. (If you use whole eggs instead of egg whites, bake at 400°F.)

Step 8

Loosen the edges of the frittata with a rubber spatula, and then invert onto a plate.