How to Make It
Heat oven to 350° F. Toss the walnuts, butter, brown sugar, cayenne, and a pinch of salt on a rimmed baking sheet. Roast, tossing once, until golden and glazed, 8 to 10 minutes.
Meanwhile, heat the oil in a large pot over medium heat. Add the onion, 1 teaspoon salt, and ½ teaspoon black pepper. Cook, stirring occasionally, until the onion is tender, 6 to 8 minutes. Add the broth, celery root, and apple, cover, and bring to a boil. Reduce heat and simmer until the vegetables are tender, 12 to 14 minutes. Puree the soup, in batches if necessary, until smooth. (A regular or immersion blender will both work.) Return to the pot and stir in the cream.
Serve the soup topped with the nuts.