3 lbs sweet potatoes
1 T olive oil
1 onion, chopped
1 celery stalk, sliced
1 apple (such as Granny Smith or Empire), peeled and thinly sliced
Kosher salt and black pepper
Heat oven to 400 degrees F. Prick the potatoes with a fork, place on a baking sheet, and roast until tender, 40 to 45 minutes.
Meanwhile, heat the oil in a large saucepan over medium-high heat. Add the onion, celery, and apple and cook, stirring occasionally, until soft, 10 to 12 minutes.
Halve the potatoes, scoop out the flesh, and add to the saucepan. Add 6 cups water, 2 t salt, and ½ t pepper. Cook until heated through, 8 to 10 minutes.
Puree the soup in a saucepan using a handheld immersion blender (Or, working in batches, a standard blender). Add water, if necessary, to reach the desired consistency. (I use vegetable broth.)