Days of fast and abstinence, when Roman Catholics eat no meat, have always tested the mettle of cooks. This soup is a favorite Lenten dish among the Roman Catholic families of New Orleans.
1. Wash the lettuce briefly under cold running water, remove the wilted outer leaves and cut each head into quarters. Shred the lettuce by first cutting out the cores and then slicing each of the quarters crosswise into 1/8-inch-wide strips.
2. In a heavy 6- to 8-quart casserole, melt the butter over moderate heat. Add the onions and, stirring frequently, cook for about 5 minutes, or until they are soft and translucent but not brown.
3. Add the shredded lettuce, water, peas, sugar and salt and, still stirring, bring to a boil over high heat. Reduce the heat to low and simmer, partially covered, for 1½ hours, until all the vegetables are very soft.
4. Puree the entire contents of the casserole through the medium blade of a food mill and discard the coarse and stringy pulp. Return the vegetable puree to the casserole and stir over moderate heat until the soup comes to a simmer. Taste for seasoning.
5. Ladle the soup into a heated tureen or individual bowls and serve at once, garnished with the fresh mint. Or, if you prefer, cool the soup to room temperature, refrigerate it for at least 4 hours, and serve it chilled and garnished with the mint.
