Preheat oven to 375 degrees F. Line a 9x13-inch baking pan with parchment paper.
Make the Crumb: In a large bowl, whisk together the flour, sugar, baking powder and salt. Use a fork or pastry cutter to cut in the butter until it is the consistency of wet sand. Remove 1½ cups of the crumb mixture and set aside. Add the egg to the remaining mixture (the dough will be crumbly). Press the egg mixture into the bottom of the pan.
Make the Filling: In a separate large bowl, whisk together the sugar, cornstarch and lemon juice. Stir in the blueberries to coat evenly with the sugar mixture. Spread the blueberry mixture evenly over the crust, drizzling any excess juice in the bowl over the filling. With your fingers crumble the mixture you set aside evenly over the blueberry layer.
Bake for 40 to 45 minutes, or until the top is light golden brown. Cool completely before cutting into squares. Leftovers can be stored in an airtight container or wrapped in plastic wrap at room temperature or in the refrigerator for up to 4 days.
Recipe by Chateau Elma at https://www.chateauelma.com/blueberry-oatmeal-crumb-bars