Preheat the oven to 350ºF and line a 9x9 in square pan with parchment paper.
In a medium bowl, mix the dry ingredients together (oats, flour, brown sugar, cinnamon, cardamom, salt).
Then knead in the butter until the mixture becomes crumbly. Add 1 Tbsp of milk at a time until the mixture barely holds together. Err on side of dry.
In another bowl, mix together the filling (berries, lemon juice, and white sugar). Mash them a little bit to release the juices.
Spread out 1/2 of the oat-flour mixture onto the bottom of the pan and press it down. Then layer the berry mixture on top. And finally, loosely sprinkle the rest of the crumb mixture on top. It's okay if some of the berries are visible from the top.
Bake for 40 min, until the topping become golden and filling is bubbly. Cool to room temperature before cutting. Enjoy!