Preheat oven to 375° F. Grease an 8-inch baking dish (I used a 6x10-inch dish).
Toss together the blueberries, sugar, cornstarch, and lemon juice. Dump into the prepared baking dish.
In a bowl, combine the flour, oats, almonds, baking powder, and salt. In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar together until smooth and creamy, about 3 minutes. Add the flour mixture and mix until combined. Clump the mixture together with your hands and drop on top of the blueberry mixture.
Bake until topping is golden and juices are bubbling, about 1 hour. If the topping browns tot quickly, cover with aluminum foil. Let sit 20 minutes before serving. Store in the fridge for up to 3 days.