Roast squash until soft and cooked through. Scrape out as much flesh as possible.
Peel and dice onion and apple. Add butter to large pot, heat to melt, then sauté onion until golden. Add apple and curry powder and cook slightly.
Pour in broth. Cut sage leaves into strips and add to pot. Cover and simmer about 5 minutes. Add squash and cook another 15 minutes.
Taste and season with salt and pepper.
Remove from heat. Carefully purée with immersion blender until very smooth. Or, pour in batches into blender and purée. Return to burner and heat through if mixture has cooled if using blender.
While soup is simmering, slice sage leaves into strips. Heat butter until bubbling, then add sage and fry quickly without browning. Remove to paper lined plate.
Lade soup into bowls and top with crispy sage leaves.