How to Make It
In a very large saucepan, combine the lima beans with 8 cups of the water and bring to a boil. Add the bay leaves and thyme sprigs and simmer over moderately low heat, stirring occasionally, until the water is almost absorbed, about 1 1/2 hours. Add the remaining 11 cups of water and simmer, stirring occasionally, until the beans are very tender, about 2 hours longer.
In a large skillet, heat 2 tablespoons of the olive oil. Add the garlic and cook over moderate heat until golden, 3 minutes. Add the cauliflower and cook, stirring, until well coated with the garlic and just starting to soften, 3 minutes. Add the flour and toss to coat. Stir the cauliflower into the soup, cover partially and continue to cook over moderately low heat, stirring occasionally, until the cauliflower is tender, 15 minutes.
Meanwhile, in a small skillet, heat the remaining 2 tablespoons of olive oil. Add the shallots and chopped thyme and cook over moderately low heat until softened, about 6 minutes. Add the cumin and cook, stirring, until fragrant, about 1 minute. Stir the mixture into the soup; season with salt and pepper.
In a small bowl, stir the fish sauce with the chiles. Discard the bay leaves and thyme sprigs and ladle the soup into bowls. Serve the sauce on the side.