Put a small plate in the fridge.
Put the strawberries in a saucepan and crush them with a potato masher or fork. Add the sugar and heat gently, stirring with a wooden spoon until the sugar has dissolved.
Turn up the heat and bring to the boil, then cook until you get a thick, sticky jam-like mixture. (The time depends on the ripeness of the raspberries and the amount of sugar you’ve added). Stir occasionally with a wooden spoon.
Take the plate out of the fridge. Drop of little jam onto it, then put it in the fridge for 30 seconds. Run your finger through the drop - if it forms a crinkle and is gluey it’s done. If not, cook and retest until it’s ready.
Discard any scum with a spoon, then mix in in the vanilla paste and lemon juice.
Spoon the jam into a small (about 200 ml) sterilised jar and seal with a lid immediately.
Let it cool then store it a cool place (such as larder, cellar etc). After opening keep it in the fridge.