In a heavy bottom pot, over medium heat, add butter.
Add squash, potato and apple on low heat, covered, stirring occasionally, until softened but not browned, 12 to 15 minutes. Add 2 Tbsp (30ml) sage and cook, uncovered, stirring, 1 minute.
Add stock, water and bay leaf to squash mixture, simmer, uncovered, stirring occasionally, until vegetables are very tender, 25 to 30 minutes.
While soup is heating, cut remaining apple into thin matchsticks for garnish.
Heat a small frying pan with olive oil on medium high and add remaining sage leaves. Fry for 30 seconds, remove on to a paper towel lined plate.
Puree soup in 4 batches in a blender, then heat in cleaned pot over medium low heat, stirring occasionally, season with salt. (Add additional ½ cup (125ml) water to thin, if necessary.)
Serve soup topped with fresh black pepper, apple matchsticks and crispy sage leaves.