Take four 7.5cm/3in pastry rings and tightly stretch cling film over the top of each one, then turn them upside down; these will become the frames for the tarts. Line a tray with greaseproof paper, place the pastry rings on the tray with the cling film on the bottom, line the insides with the acetate plastic and transfer to the freezer.
For the sorbet, in a large bowl macerate the strawberries with the stevia and lemon juice for 30 minutes. Macerating the strawberries will give the sorbet much more flavour.
Blend the strawberries to a purÃ©e. Set aside 150g/5oz of the purÃ©e and pour the remainder into a squeezy plastic bottle. Squeeze the purÃ©e into the bottom of each frozen pastry ring until you have 5mm layer covering the bottom. Return the tray to the freezer and freeze for 30 minutes, or until frozen solid.
Remove from the freezer and place a 6cm/2Â½in pastry ring centrally on top of each circle of frozen purÃ©e. Gently press it down so that it just indents the base. Squeeze more purÃ©e into the gap between the two rings, to create a wall 2cm/Â¾in high.
Place back in the freezer and leave to freeze for at least 2 hours, or until solid. (The steps up to this stage can be done a few days in advance.) Place 4 serving plates in the freezer 1 hour prior to serving.
For the tart filling, in a bowl macerate the strawberries with the stevia for 30 minutes.
Meanwhile, make the crÃ¨me Chantilly by whipping the cream with the stevia and vanilla purÃ©e or extract, until firm peaks form when the whisk is removed. (Beware, if you over-whip the cream, it will separate and you will create butter!)
Mix 100g/3Â½oz of the reserved purÃ©e with the macerated strawberries. Fold half the macerated strawberries through the Chantilly cream. Reserve the other half for decorating.
Take one sorbet out of the freezer. Remove the inner pastry ring by rubbing your fingers around the inside to soften the sorbet behind it slightly and enable you to gently lift off the ring. Now lift off the outer pastry ring and gently peel away the acetate surrounding the sorbet. Return each sorbet case to a baking tray in the freezer as you repeat this process for the remaining sorbets.
When ready to serve, remove the plates from the freezer and place a small amount of the crushed meringue in the centre of each plate to stop the tarts from sliding around, then cover with a frozen sorbet case.
Place a spoonful of the jam in the bottom of each sorbet case, followed by the strawberry and Chantilly cream mix. Finish each tart with a spoonful of the reserved macerated strawberries, followed by a few whole meringues and mint leaves. Drizzle the remaining reserved purÃ©e around each tart, place a few halved strawberries on the plates and serve immediately.