Cut the squash into slices, peel it and dice it. You will need 1 kilo of squash pulp. Put it in a pot and cook it over low heat with lemon juice and 300 ml of water for about 35 minutes, until it softens. Stir often to prevent it sticking on the bottom. Depending on the texture of the squash you might need more or less water, more or less cooking time.
Reduce the squash in a velvety puree with an immersion blender.
Add 300 g of sugar and the split open vanilla bean and simmer the jam for about 10 minutes to thicken it.
When the jam is almost ready add the sliced almonds and cook for 5 more minutes.
Spoon the jam into sterilized jars, close them tightly and boil the jars for 30 minutes to preserve the jam for months.