Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Line a baking tray with baking paper. Knead the flour, 30 g butter, 25 g sugar, oat flakes and 1 teaspoon water into crumbles and sprinkle onto the baking tray. Bake in a hot oven for 10-12 minutes and remove.
In the meantime wash and dry apples, cut them in half lengthwise. Place the halves on the cut surface and cut in deeply with a knife, but do not cut through. Sprinkle with lemon juice. Put the apples with the cut surface on the baking tray, put 1 teaspoon butter in pieces on each half of the apple. Mix cinnamon and 1 tbsp. sugar and sprinkle the apples with it. Bake in a hot oven for about 25 minutes.
Remove the apples. Spread crumbles on the apples, dust with icing sugar and arrange with 1 scoop of vanilla ice cream on each. Serve immediately.