Pre heat your oven to 400 degrees.
Heat your cast iron or heavy skillet on the stovetop to medium for about 5 minutes, add 2 TB olive oil, and then your leeks, and a sprinkle of salt and pepper. Saute for 10-15 minutes until translucent and light golden brown. Remove the leeks to a separate bowl and set aside. Turn the heat up under your skillet to medium high, add another 1-2 TB olive oil, wait 10-15 seconds, then add entire contents of hash browns and spread evenly over the bottom of the pan.
Turn heat down to medium and let the potatoes brown and crisp, without disturbing, for about 5 minutes.
In the meantime, combine the eggs and egg whites, with half and half, and herbs, and mix vigorously with a fork or whisk until combined.
Turn the hash browns with a spatula and press back into the bottom of the hot pan forming a crust. Spread leeks evenly over the potatoes, then pour egg mixture on top. Top with grated cheese and place in the preheated oven for 10-15 minutes until a knife in the center comes out clean. For more browning you can broil the top for about a minute before removing from the oven.
Let the frittata set in the pan for about 5 minutes. Serve wedges topped with a sprinkling of grated cheese and some chopped herbs.