For the streusel: Combine flour and sugar. Knead in butter to form a crumbly dough. Preheat the oven to 200°C (approximately 400°F).
For the apples: Rinse, core, quarter and slice apples. Sprinkle with lemon juice. Place in a buttered baking dish and top with streusel. Bake until golden brown, about 25 minutes. Remove and serve with vanilla ice cream.