Start by making the pastry. Cut the butter into small cubes and place the flour into a bowl. Gently toss the butter in the flour until they are completely coated.
Mix the lemon juice with 200ml cold water in a small jug. Add the liquid to the butter and flour a little at a time, mixing with a round-ended knife until a rough dough forms.
You might not need all the water. You should have a dough with large lumps of butter still showing. Wrap in clingfilm and rest in the fridge for 20 minutes.
Heat a small amount of olive oil in a pan, then add the onion and garlic. Fry over a medium heat until softened, then add the chorizo cubes, paprika and chipotle and cook for a further 2 minutes. Set aside to cool.
Take the pastry dough out of the fridge and tip onto a floured surface. Flatten the dough and roll it out into a narrow rectangle of about 2.5cm thickness.
If the dough starts to stick, add a bit more flour to the surface. Fold one third of the dough up on itself, then the opposite third down over it, a bit like folding a letter into three.
Wrap in clingfilm and chill for 20 minutes. Repeat this process a further three times, turning the pastry 90º each time.
When the onions are cool, use a food processor to pulse the mixture until no large lumps of chorizo remain. Add the sausagemeat and pulse gently to combine.
Preheat the oven to 200ºC, gas mark 6. Roll out the pastry into a large rectangle, about 40x30cm, and trim the edges. Divide into 8 even pieces, about 5x10cm.
Divide the sausagemeat mixture into 8 balls and coat each one in flour. Roll into sausage shapes 5cm long.
Brush the pastry rectangles with beaten egg, and then lay a sausage at the top end. Roll the pastry from the top down to form the sausage roll. Place onto a prepared baking tray and repeat for the remaining 7 rolls.
Brush the tops of the sausage rolls with the remaining beaten egg, and then make several diagonal cuts on the top of the pastry. Sprinkle with poppy seeds and paprika, then bake in the oven for 30 minutes, or until golden brown and cooked through.