Defrost 500 g blueberries. Place in a large pot with lime juice and jam sugar, heat while stirring. Let everything simmer for about 3 minutes while stirring. Stir in 100 g frozen blueberries.
Pour into a bowl and let it cool down.
Melt butter for the dough. Mix flour, brown sugar, salt and oat flakes in a bowl. Add butter, first with the dough hooks of the hand mixer, then knead with your hands to form crumbles.
Grease a tart mould (26 cm Ø). Put approx. 2/3 of the crumbles into the mould, press to a flat bottom, while pulling up the edge. Place the mould in a cold place. Chop the nuts very finely and mix into the remaining crumbles.
Put the cooled blueberries into the mould and smooth them down. Spread the rest of the crumbles on top. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) on the bottom shelf for 25-30 minutes.
Let it cool down on a cake rack. Serve dusted with icing sugar. Whipped cream tastes good with it.