First of all make the lemon posset mixture: Place double cream and sugar into a deep, medium large saucepan over a medium heat.
Whisk the mixture gently so as not to whip the cream. You want to ensure there are no lumps of sugar as the mixture heats. Bring to a boil, still whisking continuously, then turn up the heat to high and boil vigorously for 2-3 minutes. This is the part where it shows how much you need a deep pan as the mixture will rise A LOT!
Once you've boiled your mixture, while stirring, for 2-3 minutes, remove from the heat. You will notice that while it hasn't massively thickened, it will coat the back of a spoon and the texture will have changed. It will look a bit more saucey and like it's thickened ever so slightly.
Leave the mixture to settle for 15-20 seconds, then add the lemon juice. Stir to make sure everything is combined and then divide the mixture between 4 individual sized glass jars/serving dishes.
Leave the lemon posset mixture to cool to room temperature before placing in the fridge to set completely. This will take anywhere from 2-4 hours.
While the lemon possets are setting, make the strawberry sauce: Place diced strawberries, sugar, vanilla extract, salt cornflour, water and lemon juice into a medium sized saucepan over a low heat.
Stir the strawberry sauce continually so it doesn't stick to the saucepan and bring to a gentle boil. You will notice that the mixture will change from a milky pink to a deeper red as it boils and thickens. While boiling, you can take a potato masher and mash the strawberries as much or as little as you like. If you prefer smooth, mash them a lot, if you prefer chunky, leave them. I like somewhere in the middle, so mashed enough so that there were some smaller and some larger chunks of strawberry.
Once the mixture has thickened remove from the heat and leave to cool completely.
Once the mixture is cool and the lemon possets are set, spoon a little sauce onto the top of each posset. Garnish with a little lemon zest and a strawberry, if desired.
Lemon Possets will keep in the fridge, covered, for 2 days. Best served chilled.