Β½ lemon, juice only
700²µ/1Β½±τ²ϊ rhubarb, trimmed, chopped into 2.5cm/1in pieces
400g/14oz plain flour
200g/7oz golden caster sugar
2 tbsp golden caster sugar, plus extra for sprinkling
pinch salt
250g/9oz cold unsalted butter, cut into small chunks
knob of unsalted butter