Grate the zucchini and transfer it to a colander. Sprinkle with sea salt and allow zucchini to sit at least 1 hour (up to 3) to allow it to drain. Squeeze any remaining water out of the zucchini

Preheat the oven to 375 degrees F, and add the oil to a 10-inch cast iron skillet. Spread the oil around the full surface and sides of the skillet (I use a paper towel). Place skillet on the stove top over medium heat.

Place the arugula in the heated skillet and cover. Cook, stirring occasionally just until wilted, about 1 to 2 minutes. Remove from heat and allow arugula to cool. 

Whisk the eggs, feta, and sea salt together in a mixing bowl. Add the grated zucchini, arugula, and chopped fresh herbs. Stir together until everything is well combined. Pour the egg and veggie mixture into the cast iron skillet and place on the center rack of the preheated oven. Bake 25 to 30 minutes, or until the center has set up and the top is beginning to turn golden-brown. Remove from the oven and allow frittata to rest 10 minutes before cutting and serving.