STEP 1: Heat oven to 400° F.
STEP 2: Prick the potatoes with a fork, place on a baking sheet, and roast until tender, 40 to 45 minutes.
STEP 3: 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.
STEP 4: Halve the potatoes, scoop out the flesh, and add to the saucepan. Add 6 cups water, 2 teaspoons salt, and 1/2 teaspoon pepper. Cook until heated through, 8 to 10 minutes.
STEP 5: Puree the soup in the saucepan using a handheld immersion blender (or, working in batches, a standard blender). Add water, if necessary, to reach the desired consistency.