oven-proof glass pie pan and trim away all but 1 cm of the excess dough. Turn the excess dough under and flute the edge of the pie. Chill while preparing the filling. MIXING THE FILLING: Rinse and pick over the blueberries and drain them on a paper-towel-lined pan. Combine one third of the berries and the sugar in a saucepan and bring to a simmer over medium heat, stirring occasionally. Simmer the berries in the juices that accumulate about 5 minutes. Strain the juices into another pan. Pour the water into a small bowl and stir in the cornstarch to dissolve it. Return the blueberry juices to a boil and beat about one quarter of it into the dissolved cornstarch. Return the remaining juices to a boil and beat the cornstarch mixture into it. Return the juices to a boil, beating constantly, and allow to boil about 1 to 2 minutes, beating constantly. Stir in the remaining blueberries, the cooked berries, lemon zest and walnuts, and cool. MIXING THE CRUMB TOPPING: Cream the butter until soft, add the sugar and cream until soft and light. Beat in the cinnamon. Mix in the flour, then the walnuts. The mixture should fall into large, soft crumbs. Pour the filling into the prepared pan and smooth. Scatter over the crumbs and bake at 180ºC until the filling is set, the crumbs have colored and the crust is baked through, about 40 minutes. Cool on a rack. Makes one 23 cm pie.