1. Sift the flour into a mixing bowl. Add a pinch of salt and whisk to combine. Dice the butter and add to the bowl. Use your fingertips to rub the butter into the flour till you have a sandy mixture. If you have a food processor you can use that to combine the flour and butter. Add 1 tbsp milk to the bowl and stir with a fork to start to bring the pastry together. Add another 1 tbsp milk and stir. This may be enough to bring it together. If not, add a little more milk and stir till you have a dough. Turn the pastry out of the bowl. Gently knead to form a round. Wrap in beeswax paper and chill for 1 hour or overnight.
2. Dust your work surface with a little flour and then roll out the pastry to fit a 36cm x 12cm rectangular, loose-bottomed tart tin. Carefully lay the pastry in the tart tin and gently press it into the tin, trimming off any excess pastry. Chill for 30 mins-1 hr. Wrap and save any trimmings in the fridge, in case you need to patch up the tart case during baking.
3. Preheat your oven to 180°C/Fan 160°C/Gas 4. Lightly prick the pastry all over with a fork. Lay a piece of greaseproof or baking paper in the pastry-lined tin and fill the tart case with ceramic baking beans, or dried rice, lentils, beans or pasta. Make sure you fill the tart tin up. Slide the pastry case into the oven and bake for 30 mins till pale golden.
4. While the pastry case bakes, crack the egg into a bowl or mug and beat it with a fork. When the pastry case is golden, lift the paper with the baking beans out of the tart tin. If any holes have appeared, patch them with pastry from the trimmings. Brush the pastry case all over with the beaten egg and return to the oven. Bake for another 10-15 mins till golden and crisp. Set aside to cool.
5. Make the filling for your tart by whisking the double cream till it thickens and soft peaks form. Add the mascarpone and the honey. Finely grate in the lemon zest and squeeze in the juice from half the lemon. Gently fold together. Taste and add a little more honey, if you think it needs it (remember the fruit topping will be sweet). Chill the filling till you’re ready to assemble the tart.
6. When the pastry case has cooled in the tin and you’re ready to assemble the tart, carefully lift it out and transfer to a serving plate or board.
7. Prepare the fruit. Pop the apricot and nectarine/peach in a heatproof bowl. Boil your kettle, pour over enough hot water to cover them then leave for 1-2 mins. Drain, and gently peel off the skins. Slice the apricot and nectarine into thin wedges. If you're using mango, peel and dice the flesh, discarding the stone. Hull and slice the strawberries. Slice the skin off the kiwi fruit, then slice it into rounds.
8. Spoon the mascarpone filling into the pastry case. Arrange the fruit on top on a rainbow pattern. Stir the gooseberry jam with 1-2 tbsp warm water to thin it, then lightly brush it over the fruit (this is optional, but it makes the fruit glossy and extra sweet). Serve the tart in slices. It will keep in the fridge for up to 2 days.