In a saucepan heat the olive oil. When hot, add the onion, cover and simmer gently for about 3 to 4 minutes or until tender. Add the lettuce and stir just until wilted. Add the 2 cups of water and peas and bring just to a boil and cook for 3 to 4 minutes. Remove from heat and season with salt and pepper. Puree through a food mill or in a food processor with the mint, then add enough ice water or cream to thin to the desired consistency. Chill until serving time. Before serving, adjust the seasoning and serve garnished with chives and yogurt if you wish.