Prep: Preheat your oven to 375 degrees F. Get out a deep 9-inch pie dish.
Make the Blueberry Filing: Stir together all blueberry filling ingredients in a large mixing bowl or in your baking dish. If the filling is a bit too much for the baking dish you have, you can always add a bit to a small ramekin and bake it alongside the big dish.
Make the Topping: In a medium bowl, whisk together the flour, sugar, oats, salt, and cinnamon. Cut the butter into large chunks and add it to the dry ingredients. Cut the butter into the flour using a pastry cutter or clean hands, until you have a crumbly mixture with pieces of butter the size of blueberries (some will be smaller). Give the filling one last stir, and sprinkle this evenly over the top of the blueberry filling. (Add a handful or two to your extra ramekin if you went that route).
Bake: The filling will be very bubbly, so I recommend putting down a piece of foil on the rack below the rack you bake this on, to catch drips. Bake for 52 to 57 minutes. The topping will be golden brown and the filling will be very bubbly.
Serve + Store: Let rest for 5-10 minutes before serving, and enjoy with vanilla ice cream! This is best when it's fresh out of the oven, but leftovers can be covered and kept in the fridge for a 3-4 days. Reheat gently before eating leftovers, if desired.