Make the pastry dough and chill it. Preheat the oven to 350 degrees. Meanwhile, place the blueberries in a pan over medium heat with the lemon juice and sugar. Cook until soft, tasting for sweetness. If there is too much juice, pour some off and save it to make a fruit sauce later. Roll out the pastry dough into a circle an inch larger than a 9-inch cast-iron or enameled cast-iron skillet. Place the circle on top of the pan and press down the edges with the blade of a butter knife. Brush with milk or egg yolk and bake for 20 to 30 minutes, or until the pastry is golden brown.