Separate the leaves of the escarole and discard any that are bruised. Cut them into pieces about 2 inches square. Wash well in a bowl of water.
Heat 2 tbsps of the olive oil in a wide frying pan and cook the onion over a high heat, stirring continually, for about two minutes.
Add the garlic, escarole and a sprinkling of salt. Lower the heat and cook until the greens are wilted aqnd the stems are fairly tender. This will take about five minutes.
Season to taste with freshly ground black pepper, more salt if needed and a little vinegar to sharpen the flavours.
Beat the eggs in a bowl then stir in the cheese and the cooked escarole.
Heat the remaining tablespoon of oil in a 10 inch frying pan, tip the pan to coat the surface then quickly pour in the egg mixture. Loosely cover the pan and cook the eggs over a low heat until they are nearly set.
Remove the lid and finish cooking under the grill until browned- about 2-3 minutes. Turn the frittata out onto a platter with the golden brown side facing up. Serve hot, warm or at room temperature.