Heat the olive oil and/or butter in 10-inch cast iron or nonstick skillet over medium heat. Add the onion or leek, and cook, stirring, until tender, about 4-5 minutes. Add the garlic and cook, stirring, until fragrant, about 30 seconds. Pile in the greens, and cook, stirring occasionally, until wilted. Add the cooked vegetables, and season with salt and pepper. Pour in the eggs. Cook, stirring, until the eggs start to set along the bottom and edges. Sprinkle the top of the frittata with the cheese(s) of your choice (the version above was sprinkled with Parmesan and dolloped with fresh ricotta).