Squeeze the spinach dry and finely chop.

In a 9-inch skillet, heat the olive oil over medium heat. Add the onions and cook, stirring, until soft, about 5 minutes.

In a mixing bowl, combine the eggs, spinach, salt and pepper, to taste. Add the cheeses and mix well to combine. Pour this mixture into the skillet and cook until the bottom has set, about 5 minutes.

Holding a flat lid over the skillet, turn the frittata over onto the other side and slide back into the skillet. Cook 5 minutes more and serve immediately.