Remove your pie crusts from the refrigerator about 15 minutes before you need them. If you're using store-bought crusts, let them sit at room temperature. If you've made your own (which I always encourage), they should be properly chilled. Cold dough is essential—it prevents your crust from shrinking during baking and helps create those flaky layers we all love. While your dough comes to temperature, preheat your oven to 375°F (190°C). Place one crust into a 9-inch pie dish, letting it settle naturally without stretching. You want it to fit comfortably without stress.

In a small bowl, whisk together your cornstarch, sugar, salt, and cinnamon (if using). This step is important because it distributes the cornstarch evenly throughout your sugar mixture, preventing lumps when you add it to your berries. Take your time here—clumpy cornstarch will create lumps in your filling. You can also do this step directly in your cooking pan, but I prefer mixing it separately so I know everything is properly combined before heat comes into play.

Pour your frozen blueberries into a large saucepan. Don't add the butter yet—we'll do that at the end. Sprinkle your cornstarch mixture over the berries, then add the lemon juice and lemon zest. Now here's the critical part: place this pan over very low heat and stir gently and consistently for about 3 to 5 minutes. You're not trying to cook the berries—you're just beginning to wake them up. As the temperature slowly rises, the berries will start releasing their beautiful juices. This gentle start prevents the cornstarch from cooking too quickly at the bottom of the pan. Stir often, reaching into the bottom corners where heat collects.

Once you can see that the berries have released some liquid and the mixture is starting to warm (after about 5 minutes on low), turn your heat up to medium. Continue stirring regularly—and I mean regularly. This is not the time to check your phone or step away. The cornstarch needs constant attention to dissolve properly and thicken evenly. You're looking for the mixture to begin bubbling around the edges, which usually takes 5 to 7 minutes depending on your stove. Keep your wooden spoon moving through the mixture, scraping the bottom and sides.

Once you see small bubbles beginning to form, let the mixture bubble gently while you continue stirring for exactly 1 full minute. This is where the magic happens. The cornstarch is now fully activated by heat, and it's thickening your berry liquid. You'll notice the mixture transforms from cloudy to clear and glossy—this is your visual cue that the cornstarch has done its job. The filling will look almost jewel-like when you lift your spoon. If it still looks cloudy and thin, give it another 30 seconds of boiling, but usually 1 minute is perfect.

Remove your pan from the heat immediately after that final minute of boiling. Stir in your tablespoon of butter if you're using it. The residual heat will melt it perfectly, and it adds a subtle richness that makes people ask what your secret ingredient is. Pour this gorgeous filling into a large bowl or shallow dish. Let it cool completely to room temperature—this is non-negotiable. If you pour hot filling into your pie crust, the bottom will turn soggy and the eggs in your crust won't set properly. I usually give it 45 minutes to an hour. If you're short on time, you can make this filling 1 or 2 days ahead and refrigerate it in an airtight container. Cold filling is actually easier to work with when you're assembling your pie.

Once your filling is completely cool, pour it into your waiting bottom crust. Don't overfill—leave about ¼ inch of space at the top. Any excess filling can be gently warmed and served alongside your pie. Place your second pie crust on top, either as a traditional full crust or cut into strips for a lattice pattern if you're feeling creative. Trim the edges so they're even, then crimp them firmly between your thumb and forefinger. This seals the crusts together and creates that beautiful finished look.

In a small bowl, whisk together your egg and 1 tablespoon of water to create an egg wash. Brush this all over your top crust with a pastry brush, being generous but not sloppy. This creates that beautiful golden-brown finish everyone loves. If you have coarse sugar, sprinkle a little over the top now. Make 4 small slits in the top crust with a sharp knife to allow steam to escape during baking—this helps prevent your crust from puffing up unevenly.

Place your pie on a baking sheet (this catches any drips and makes cleanup easier) and bake at 375°F (190°C) for 40 to 50 minutes. You're looking for a crust that's deep golden brown, not pale or dark. Halfway through baking, if the crust is browning faster than the filling is cooking (you'll see it browning too quickly), tent it loosely with aluminum foil. Let the pie cool on a wire rack for at least 2 hours before slicing. This allows the filling to set completely and makes cutting clean, beautiful slices possible.