Cook the onions in the oil until translucent. Season with salt and pepper. Then place into a skillet or an oven proof dish, alongside the pre-cooked vegetables, and salmon. Sprinkle the cheese over the mixture, and then pour in the beaten eggs. Stir well, and use a spatula to evenly distribute the ingredients over the bottom of the dish.
Continue cooking the frittata, without stirring, over the burner for an additional 45-60 seconds. Just until the outside edge of the frittata begins to set.
Carefully place the hot dish into the oven and bake for 10-14 minutes, just until the eggs are set. Let the frittata set for at least 5 minutes before cutting.