Grease an ovenproof dish (20 x 30 cm). For the filling, wash lemon hot, dab dry and finely grate the peel. Squeeze the lemon. Select the blueberries, wash and drain well.
Mix with lemon peel, lemon juice, 4 tablespoons sugar, 3 tablespoons flour and 1⁄2 tsp cinnamon. Fill fruits into the form. Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer).
For the basic crumbles, knead 150 g flour, 75 g sugar, 75 g butter in pieces and 1 pinch of salt with the dough hooks of the mixer to form crumbles. Coarsely chop the almonds. Mix with oat flakes into the crumbles.
Spread on the blueberries. Bake in a hot oven for about 40 minutes. Take out and let cool down.