ROUGHLY CHOP VEGETABLES into 1 cm pieces and set aside on a plate. Heat the oil in a large skillet over medium heat, add the onion and cook, stirring, for 5 minutes. Decrease heat to low, add the vegetables and cook for 10 minutes. Remove vegetables from heat and scrape the mixture into a food processor. Add the egg whites and bread crumbs and puree until smooth. Stuff the mixture into sausage casings or form 2 long sausages in plastic wrap. Poach the sausages in gently simmering water for 7 minutes, drain and let cool. If you have used plastic wrap to form the sausages, remove it when sausages are cool. Cut sausages into 15 cm lengths. To serve, grill the sausages or place under a preheated broiler. Since the sausages are already cooked, we want only to reheat them and crisp their skins.