Make the streusel first. In a small bowl whisk together the half cup of oat flour, brown sugar, granulated sugar, and cinnamon until evenly combined. Add the 20 grams of cold salted butter and use your fingertips to work it into the dry mixture, pressing and rubbing until the butter is fully incorporated and the mixture looks like coarse, sandy crumbs with some pea sized pieces remaining. Set aside in the fridge while you prepare the batter.
In a large bowl whisk together the 2 and a half cups of oat flour and granulated sugar and salt.
In a separate bowl or large measuring cup whisk together the eggs, reduced fat buttermilk, unsweetened almond milk, melted salted butter, lemon juice, and lemon zest until combined.
Pour the wet mixture into the dry mixture and stir until just barely combined. The batter should still be lumpy. Do not overmix.
Optional overnight step: cover the bowl and refrigerate the batter overnight. This deepens the flavor and gives the casserole a slightly more tender texture. If you are making this the same day skip this step and proceed.
When ready to bake preheat the oven to 350°F. Generously coat a 9x13 inch baking dish with butter.
In a small bowl whisk together the remaining half cup of oat flour with the baking powder and baking soda. Add this mixture to the batter and stir until just combined.
Pour the batter into the prepared baking dish and spread it evenly. Scatter the raw blueberries evenly over the top, then crumble the streusel over everything in an even layer.
Bake for 45 to 50 minutes until the edges pull away from the sides of the dish and a toothpick inserted into the center comes out clean. Remove from the oven and allow to cool for 5 minutes before slicing and serving.