Defrost 500 g blueberries. Mix well with lime juice and jam sugar in a large pot and bring to the boil while stirring. Cook for at least 3 minutes while stirring until bubbly. Stir in 100 g frozen blueberries.
Pour into a bowl and leave to cool.
For the dough, melt 250 g butter. Grease a tart mould with a lifting base (26 cm Ø). Mix flour, brown sugar, 1 pinch of salt and oat flakes in a bowl. Add the liquid butter and knead briefly with the dough hooks of the mixer, then knead into crumbles with your hands.
Approx. 2⁄3 put the crumble into the mould, press it to a flat bottom and pull up one edge. Place the mould in a cold place.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Finely chop the nuts and mix with the remaining crumbles. Fill blueberries into the mould, smooth down. Spread the nut crumbles on top.
Bake in the hot oven on the lower shelf for 25-30 minutes.
Take out and let cool on a cake rack. Serve dusted with icing sugar. Serve with whipped cream.