How to Make It
In a 1- to 2-quart pan, stir together 1/4 cup sugar, the blueberries, and 1 1/4 cups water. Bring to a boil over high heat, then reduce heat to maintain a simmer, cover, and cook, stirring occasionally, until blueberries are soft and most of the liquid is absorbed, about 45 minutes. Chill until cool, about 40 minutes.
Meanwhile, in a bowl or in the bowl of a food processor, mix or whirl flour, oats, salt, and remaining 3/4 cup sugar until combined. Cut in butter or pulse until mixture forms coarse crumbs. Add egg yolk and mix or pulse until mixture comes together in a crumbly dough. Divide in half. Press half the mixture evenly over bottom of a buttered 8-inch square baking pan. Chill pan and reserved dough while filling cools.
Spread blueberry mixture evenly over crust in pan, then sprinkle and pat the remaining half of the dough evenly over the blueberry layer.
Bake in a 350º; regular or convection oven until top is browned and filling is bubbling around the edges, 35 to 40 minutes. Let cool about 15 minutes, then cut into squares. Serve warm, or let cool completely.