Heat the oil or ghee in a large skillet over medium-high heat; once hot, add the onion, garlic, and red pepper flakes. Cook until the onion is translucent, about 5 minutes.
Add the lemon juice and the chopped greens (you may need to do this in batches as it cooks down). Cover the pan tightly and turn the heat to low. Keep covered, stirring occasionally, until the greens are soft, about 5 minutes. Season with salt and pepper.
Turn the broiler on high. In a large bowl, vigorously whisk together the eggs with a few pinches of salt, pepper, and the parsley or basil. Pour the eggs gently over the cooked greens and cook over medium-low heat, uncovered, until the edges of the frittata are set, about 5 minutes. Then, transfer to the oven and broil until the top is lightly browned and set, another 3 minutes or so.
Take out of the oven and serve warm, at room temp, or cold. Garnish with additional fresh herbs and salt if desired.