3/4cupgrated cheeselike Parmesan, feta, goat, or cheddar
Heat a 10-inch or larger skillet with oil. Add the onion and sauté about 2 minutes before adding the garlic. Cook another minute before adding the sausage - if it's precooked, sauté another couple minutes to heat through, if sausage isn't precooked, sauté until cooked thoroughly and no pink is showing.
Add the chard, kale, or spinach, stir a bit, and cover for about 4 minutes to steam.
Meanwhile, whisk the eggs and milk (or water) together and season with salt and pepper. Add half of the cheese and stir to mix.
Remove the cover and stir the mixture in the skillet. Let it cook a little more, allowing any liquid to evaporate. Pour the eggs evenly over the top. Do not stir - let the eggs set around the edges and somewhat in the middle.
Turn on the oven broiler to preheat.
Using a thin spatula, lift the edges and tip the pan so the liquid egg from the middle runs to the edge and under the cooked egg. Continue all the way around the edge until most of the egg is cooked.
Put the pan under the broiler (cover the handle with foil if it is not oven proof) and broil 3-5 minutes until all the egg is cooked and the top is browning (Put the tip of a knife in the middle of the frittata to see if the egg is cooked through.)
Remove from under broiler, sprinkle with remaining cheese and then broil for another minute until the cheese has melted. Serve immediately.
Store any leftovers in the fridge for 3 to 4 days.