Peel apples, halve them and remove the seeds with a melon cutter. Place apples on a board with the cut surfaces facing down. With a small knife, cut them close together lengthwise, but do not cut through them. Put the apples in an ovenproof dish. Sprinkle with lemon juice.
Mix the cinnamon with 2 tbsp. sugar and sprinkle on the apples. Put 1 tsp. butter on each apple half. In a preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see manufacturer)
Knead the flour, 25 g sugar, 40 g butter and oat flakes into crumbles. Remove the apples from the oven, spread the crumbles on the apples and bake in a hot oven at the same temperature for about 15 minutes. Remove apples from the oven. Arrange apple halves and 1 scoop of ice cream on plates.