Evenly distribute blueberry mixture over the filling.
Evenly sprinkle with the reserved heaping 3/4 cup crust mixture. Before sprinkling, I squeeze the mixture in my palm to encourage bigger crumble pieces to form.
Bake for about 60 minutes, or until edges are bubbling quite vigorously and there’s some bubbling in the center. Crumble topping should appear set and very pale golden. I used frozen berries without thawing and the pan was very cold going into the oven. If using fresh berries, baking time will be reduced, but I’m not sure by how much, I’d guess at least 10-15 minutes, possibly less. Watch your bars, not the clock, when evaluating doneness.
Place pan on a wire rack and allow bars to cool for at least 1 to 2 hours (overnight is even better) before slicing and serving. If you try to slice bars before they’ve cooled completely, they’ll be extremely messy and could fall apart. Bars will keep airtight at room temperature for up to 1 week, in the fridge for up to 10 days (they’re good slightly chilled if you like chilled pie), or in the freezer for up to 6 months.