Combine 6-7 cups of blueberries with 1 cup of sugar, 1/4 cup of cornstarch, and 1-2 tablespoons of lemon juice. Stir until well mixed.
Preheat the oven to 375°F (190°C). Fit one crust into a 10x15-inch baking sheet, leaving an overhang. Pour the blueberry filling and spread evenly.
Place the second crust over the filling. Create a lattice pattern or cut slits for ventilation. Seal the edges and brush with egg wash.
Bake for 35-45 minutes until the crust is golden brown and the filling is bubbly. Use foil to protect the edges if needed.
Cool the pie on a wire rack. Once cooled, lift out using parchment paper and cut into squares or rectangles for easy serving.
Serve warm or at room temperature with vanilla ice cream or whipped cream. Store leftovers in the refrigerator for 3-4 days. Reheat in the oven or microwave.