Preheat the oven to 180°C.
Peel and core the apples, then chop them into bite-sized chunks.
Arrange the apple pieces in baking dish, scatter the walnuts and sultanas on top, then squeeze the lemon juice over. Dust with half of the sugar and cinnamon.
Mix the flour, oats, ground walnuts and remaining sugar& cinnamon in a bowl.
Rub the butter/spread with the mixture until you get a coarse crumble.
Cover the apples evenly with the crumble mixture, then slightly press down the top with a spoon.
Bake for 20-25 minutes until the top is golden and crispy.
Serve it warm.