Put the flour and butter in a food processor and blitz until the mixture resembles breadcrumbs. Add 2-3 tbsp of cold water and pulse until the pastry starts to come together in a ball. Remove from the processor and gently knead into a disc. Wrap in clingfilm and chill for 30 minutes.
Preheat the oven to 210°C/gas mark 7. When the pastry has chilled, roll it out to a thickness of around 3mm and line 8 individual tart tins. Press the pastry into the tins, then trim the edges. Line with greaseproof paper and fill with baking beans, then blind bake for 20 minutes. Remove the beans and return the tarts to the oven for another 5 minutes to completely dry out.
Meanwhile, fry the chorizo in a frying pan over a medium heat until crisp and it's released its oil. Add the shallot and continue cooking for a couple of minutes. Remove from the heat and leave to cool slightly, then add the crab and stir well to combine. In a separate jug, whisk together the crème fraîche, milk, eggs and dill. Season with black pepper.
When the tart cases are cooked, remove from the oven and reduce the temperature to 160°C/gas mark 3. Divide the crab and chorizo mix between the tart cases, then carefully pour over the egg mixture. Carefully transfer the tarts to the oven and bake for 20 minutes, or until the egg mixture has set.