600g/1lb 5oz eating or cooking apples, peeled, cored and chopped into 2–3cm/¾–1¼in chunks
1 tbsp lemon juice
400g/14oz rhubarb, trimmed and chopped into 3cm/1¼in chunks
175g/6oz plain flour (or 125g/4½oz plain flour plus 50g/1¾oz rolled porridge oats)
75–95g/2¾–3¼oz caster sugar
125g/4½oz caster or light soft brown sugar, plus 1 tbsp for sprinkling
½ tsp ground cinnamon
100g/3½oz butter, at room temperature and cut into small pieces