Cut the pumpkin into thin slices, remove the seeds and peel the rind. Wash it and cut it into cubes with a serrated knife.
Put the pumpkin pieces in a pot, layer by layer, while sprinkling sugar in between them. Leave them to sit for 2-3 hours, for the pumpkin to release its juice.
Then add water and boil until the pumpkin pieces soften. Boil with the lid open to allow the juice to thicken.
Remove from the heat and leave it to cool. Optionally, you can sprinkle the jam with walnuts. Distribute into medium-sized jars, sterilize for 5 min.