For the crumbs: combine the oatmeal and oatmeal flakes in a bowl.
Add the chopped walnuts, cinnamon, sugar and a pinch of salt.
Add pieces of cold butter.
RUB thoroughly with your hands into a crumb.
For berry filling: add sugar and cornstarch to the blackberries, mix thoroughly and gently (I used frozen berries, did not defrost them beforehand).
In a baking dish (the size of my form is 18 x 26, the bottom is covered with parchment), put half the crumbs, flatten and slightly tamp.
Spread the BlackBerry filling on top.
Spread the remaining crumbs over the entire surface. Send to bake in a preheated 190 degree oven for 40-45 minutes (focusing on the Golden top and features of your oven). Remove the finished product from the oven and allow to cool completely.
Put the already cooled cookie stock in the refrigerator for a couple of hours (I left it overnight), then cut it into pieces (I got 12 pieces).