Preheat the oven to 375F. Warm 1 tablespoon of the oil in a 8 inch cast iron skillet or oven-safe pan over medium heat and cook, stirring often, until the leeks begin to soften and become fragrant, about 3 minutes. Stir in the chopped chard leaves and continue to cook, stirring often, until the leaves begin to wilt, about 3 minutes.
While the veggies cook whisk together the eggs, cream, spices and 1/4 cup of the cheese. Season with salt and pepper.
Remove the veggies from the pan and stir them into the egg mixture until everything is well combined. Wipe out the skillet with a towel and heat the remaining 1 tablespoon oil in the skillet over medium-low heat. Pour the egg/veggie mixture into the pan and sprinkle with the remaining feta cheese. Cook over medium-low until the edges begin to firm, about 8-10 minutes. If desired, sprinkle extra cheese over the top before transferring to the oven.
Transfer the skillet to the oven to finish cooking, about 10 minutes more, until the eggs are set and golden brown at the edges. Remove the frittata from the oven and set aside to cool for about 5 minutes before enjoying.