Preheat the oven to fan 160C/ conventional 180C/gas 4. Lightly butter two 12-hole bun tins (no need if using non-stick). Beat the butter and sugar together with a wooden spoon for a minute or two until soft and creamy. Mix in the hazelnuts.
Put a heaped teaspoon of the mixture into each tart tin. Bake for about 10 minutes until golden and risen slightly up the sides of each tin. Leave to cool in the tins for about 5 minutes or until the mixture is firm enough to remove, then ease each tart from the tins with a small knife and put on to a sheet of kitchen paper as the bases are quite buttery. (The tart cases can be made up to 1 day ahead to this stage and stored in an airtight tin in a cool place.)
For the blueberries, put the sugar in a small pan with 1 tbsp water. Put over a low heat until the sugar has dissolved, then turn up the heat a bit and bubble gently until syrupy, about 30 seconds only. Take off the heat and tip in the blueberries. Stir gently just to coat the berries in the syrup – you want to keep them whole – then leave to cool.
Beat the mascarpone with enough milk to make a soft creamy mixture, a bit like whipped cream. Lightly stir in the lemon curd so it ripples – it looks nice if it’s not fully stirred in. You now have a choice.
For a teatime treat: Spoon a little of the mascarpone mixture into each tart case, then spoon over a few of the syrupy blueberries.
For a pretty dessert: Spoon half of the syrupy blueberries into a bowl and set aside. Cook the ones left in the pan gently for a few minutes just to burst and soften the berries – you will get lots of juices. Using a wooden spoon, press them through a sieve into a bowl to make a sauce. Cool. Ripple a spoonful of the sauce into the mascarpone mixture, then spoon a little into each tart case. Pile the whole syrupy blueberries on top. Do this no more than an hour ahead – less on a warm day, as they soften. For each serving, put 3 of the tarts in the middle of a plate and spoon over a little sauce. Serve the rest of the sauce separately.