Line a square baking tin (approx. 20 x 20 cm; see below) with baking paper. Knead flour, semolina, 100 g sugar, orange peel and butter in pieces to crumbles. Press a good half of the crumbles in the tin to form a base.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Sort blackberries and wash if necessary. Mix 2 tbsp. sugar and starch. Stir smooth with orange juice and mix carefully with the berries.
Spread on the crumble dough base.
Finely chop the walnuts and knead them into the remaining crumbles. Spread on the berries. Sprinkle with brown sugar. Bake in a hot oven for about 45 minutes. Take out and let cool down. Carefully remove from the tin and cut into about 15 pieces.