I love this simple soup, but you can jazz it up by sprinkling a tablespoon of curry powder over the onion mixture before adding the sweet potatoes and apples. Stir it into the onions and let it cook for a minute or so until it is very fragrant.
3 tablespoons butter
1 large white onion, chopped
1 stalk celery, chopped
2 cloves garlic, minced
1-inch piece fresh ginger, chopped
2-1/4 pounds sweet potatoes (about 4 medium), peeled and cut into chunks
2 green apples, peeled, cored and cut into chunks
8 cups chicken or vegetable broth
Salt and pepper to taste
In a large (7-quart) Dutch oven, melt the butter over medium heat. Add the onions to the butter, stirring to coat. Cook the onions until soft and translucent, but not browned, stirring frequently, about 10 minutes.
Add the chopped celery and cook for a few more minutes until soft. Add the garlic and ginger, stir to coat and cook a further minute. Add the sweet potato chunks and the apple pieces and stir to coat, then immediately pour in the broth.
Bring the soup to a boil, then reduce the heat and simmer, covered, until the potatoes and apples are completely soft, about 30 minutes.
Using an immersion blender or very carefully in batches in a blender, puree the soup. Season with salt. The soup will keep, covered, in the fridge for a day. Reheat gently before serving.