Omelet: Make the sauce first. fry the onion in the shortening, then add the peppers, tomatoes, and puree. Season with salt and pepper to taste. Simmer for 25 minutes. For the omelet: Beat the egg whites alone, then beat the yolks alone until thick and lemon color. Add salt, flour and milk to the yolks and mix. Then fold yolk mixture into the beaten whites. Make into 5 or 6 omelets, and fry then in the fat or oil, on both sides. Remove from the frying pan when a delicate brown on both sides and spread each omelet with the sauce and roll up. Pour what is left of the sauce over the rolled omelets before serving. From The Mexican Plaza Cookbook 1971