ROUGHLY CHOP VEGETABLES into 1/4-inch 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 6-inch 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.