Put the blueberries, water and lemon juice (and vanilla, if using) in a non stick pan, cover and bring to a boil.
Remove the lid and cook on a medium heat for ca 15 minutes, stirring regularly. The mixture will reduce by ⅓ and thicken. Smash the berries with a spoon while they cook.
You can blend the mixture if you wish to have a smooth consistency. I like it the way it is, with a few bits and pieces in it.
At the end, take a couple of pinches xanthan gum and sprinkle it evenly into the pot, stirring well afterwards. Xanthan gum is a brilliant thickening agent and is also used in gluten free baking, so it does come in handy. You only need a tiny amount - it's quite remarkable.
Pour the mixture into a clean jar and store in the fridge. This recipe fills a small (200ml) jar.
A this mixture makes 200g jam, I calculated 1 portion=14g=1tbsp.Recipe Tip - How sweet your jam is will depend on the natural sweetness of your blueberries. If you find the jam is not sweet enough, feel free to add a teaspoon or two of powdered erythritol or 4-8 drops of stevia glycerite.