Heat oil over medium high in a 10" oven proof skillet, such as cast iron
Add cubed sweet potato, and cook, stirring often, until tender and slightly crisp
Add leeks and cook until softened
Remove from heat and add salmon and spinach to skillet, arranging evenly in the pan.
Sprinkle with cheese if using
add salt and pepper to beaten eggs, and pour eggs over the skillet ingredients
Bake in center of oven 15-20 minutes or until edges are pulling away from pan and middle is set.
Scatter with parsley or dill
Cut into wedges
It's important not to over beat the eggs, as incorporating too much air will cause the frittata to pouf up in the oven, then fall. which will flatten the dish and make it very dense.The nutritional information for this recipe doesn't include the optional cheese.