Why is my lemon filling runny?
Most often it did not boil long enough after adding cornstarch. The filling needs to come to a full, strong bubble and stay there for 1 to 2 minutes so the starch fully thickens. Look for cues: it should turn very thick and glossy, look a bit more translucent, and your whisk should leave visible trails. Also make sure you chill the finished pie long enough, at least 4 hours.
Why did my meringue weep or get watery?
Common causes: sugar not fully dissolved, underbaked meringue, or meringue sitting on a cooled filling. For best results, spread meringue on hot filling, make sure the sugar is dissolved (rub a bit between fingers, it should not feel gritty), and bake until the peaks are nicely set and toasted. Also, avoid tightly covering the pie in the fridge, since condensation can encourage weeping.
Can I make this with bottled lemon juice?
Fresh is better, especially for a pie this simple. Bottled can taste flat or slightly bitter. If bottled is all you have, add extra zest to help.
How do I get the meringue soft and chewy instead of dry?
Do not overbake it. Bake just until the peaks are toasted and the meringue is set. It will finish setting as it cools. Overbaking drives off too much moisture and turns it chalky.
Do I need to worry about egg safety in the meringue?
If you want a concrete target, you can spot-check the center of the meringue with an instant-read thermometer. Aim for about 160°F for peace of mind. The meringue should also look set (not wet or sliding) with toasted peaks.
My crust shrank during blind baking. How do I stop that?
Chill the shaped crust for 20 to 30 minutes before baking, use plenty of pie weights pressed up the sides, and make sure the crust is well-fitted in the corners. A little docking (a few fork pricks) on the bottom can also help.
Can I make it ahead?
You can bake it the same day and chill for several hours. If you need to prep ahead, make and blind bake the crust up to 2 days in advance and store it at room temp, wrapped well. Fill and top with meringue the day you plan to serve.