Preheat oven to 350°F.
Add oats, flour, and sugar to the melted butter. Combine until a crumble mixture forms. The texture should be similar to clumpy wet sand.
Leave a cup of the crumble mixture to the side, and press the rest down on a baking sheet lined pan.
In a large bowl combine the sugar, flour, and half the juice of a lemon with the frozen blueberries.
Layer the blueberry mixture over of the oat layer, and sprinkle the rest of the oat crumble on top.
Bake for 50 minutes or until the crumble layer is slightly golden. Cool completely before slicing into bars. Enjoy! :)
Recipe adapted from Averie Cookies