Combine onion, celery, garlic and apple juice in a large soup pot. Cover and simmer for 10 minutes.
Add squash, potato, stock, bay leaf, thyme, salt and nutmeg. Bring to a boil, cover, lower the heat and simmer for 20 to 25 minutes.
Remove the soup pot from heat, squash and potatoes should be very soft. Working in batches, puree the soup with the evaporated skimmed milk in a blender or food processor. Reheat gently. Add salt and pepper.