1 pound dried large lima beans
2 tablespoons olive oil
2 celery stalks, finely chopped
1 large carrot, finely chopped
1 leek (white and pale green parts only), finely chopped
1 large yellow onion, finely chopped
4 garlic cloves, minced
6 sage leaves, finely chopped
4 thyme sprigs
1 bay leaf
10 cups vegetable stock
2 sweet potatoes
3 tablespoons maple syrup
Salt and freshly ground black pepper
Chopped parsley, for garnish
The night before you plan to make the soup, place the lima beans in a bowl and cover with a few inches of cool water. Soak overnight, then drain and rinse the beans before starting the soup.
Preheat the oven to 400°F.
Heat the olive oil in a large pot over medium heat. Add the celery, carrot, leek, and onion, and sauté until tender. Add the garlic, sage, thyme, and bay and sauté for another minute. Add the drained lima beans and vegetable stock, and bring to a boil. Reduce the heat to medium-low and skim any foam off the top.Cover the pot with the lid slightly ajar, and simmer, stirring occasionally, until the beans are very tender, about 2 hours. Add 1 1/2 teaspoons of salt in the last half hour of cooking (or once the beans are softened but still slightly crunchy in the middle). Discard the bay leaf and thyme sprigs.
While the soup is simmering, roast the sweet potatoes. Prick the sweet potatoes all over with a fork and roast on a foil-lined baking sheet until soft, about 1 hour. When cool enough to handle, peel the sweet potatoes or slice the potatoes and scoop out the flesh, whichever you find easier.
Combine the sweet potato flesh, maple syrup, a half teaspoon of salt, and one-third of the soup (including liquid and beans) in a blender. Process until smooth. Return the purée to the pot and stir to combine. If desired, add water (or more stock) for a thinner consistency. Taste and adjust seasonings with more salt and pepper. Simmer to rewarm the soup.
Ladle the soup into bowls and garnish with parsley.