Heat the oil over medium heat. Add the sliced leek, stir and cook for a few minutes, covered.
Add the squash pieces and thyme and cook uncovered for a few minutes, stirring frequently.
Pour in the water (or stock), raise the heat, bring to a boil, then cook, covered, over medium-low heat until the squash is tender.
Puree with an immersion blender or in a blender (being careful about blending a hot liquid).
Add salt and pepper to taste.
To serve, ladle the soup into a bowl and top with a spoonful each of diced cheese and apple.