Line an 8x8 pan with parchment paper so that parchment hangs over edges.
Preheat oven to 350℉.
In a large bowl, combine oats, spelt, almond flour, brown sugar, cardamom, and salt. Stir in the melted butter.
In another bowl, toss together blueberries, cornstarch, sugar, and lemon juice.
Transfer most of the oat mixture onto the parchment paper-lined pan. (Reserve about 1/2 cup for the topping.) Spread the mixture evenly into the pan and press down firmly with your palms to compact.
Spread the blueberry mixture onto the top of the pan. Then distribute the remaining oat mixture on top. Press down a bit to make sure it sticks to the blueberries.
Bake in the oven for 38-45 minutes, or until blueberries are almost bursting and the topping is a golden brown.
Let cool before slicing. Top with raspberries if desired. Store at room temperature for up to a day or in the fridge for up to 4 days.