Step 1Heat the broiler. Bring a large pot of water to a boil for the nettles and, using tongs or rubber gloves, put them into the water and blanch them, for about 1 minute. Drain and when cool enough to handle, press out the water, chop into a rough cut and reserve.
Step 2Warm the olive oil in a 10-inch skillet. Add the garlic and onion and cook over low heat until softened, about 5 minutes. Add the nettles and season with salt and pepper.
Step 3Beat the eggs with one-half teaspoon salt, then stir the nettle and onion mixture into the bowl and add the Pecorino. Add the ricotta, leaving it a little streaky.
Step 4Wipe out the skillet and return it to the heat with the butter. When the butter has foamed then subsided, pour in the egg mixture. Stir until the ingredients are emulsified. Turn the heat to medium-low and cook until the eggs have set up around the edges and are golden on the bottom, about 6 to 7 minutes. The center will be slightly jiggly.
Step 5Slide the pan under the broiler and cook until the top is set and golden, about 1 minute. Check to see that the eggs are cooked (the frittata should be set in the center); cool slightly or to room temperature before serving.