Heat oven to 200C/180C fan/gas 6. In a bowl, mix the apples with the lemon juice, cinnamon, 1 tbsp of flour, 15g of butter and 1 tbsp of brown sugar. Place in a small glass baking dish, cover with cling film and cook in the microwave for 5 mins at 600W, or slightly less time if you have a higher wattage.
Meanwhile, rub the remaining flour, butter, sugar, oats and nuts with your fingers to a crumble-like but chunky texture.
Spoon the cooked apples into an ovenproof dish. Scatter over the crumble and bake in the oven for 20 mins until crisp. Serve with ice cream or crème fraîche.