Lemon meringue pie is a perfectly balanced dessert marked by swirling peaks of sweet, nearly weightless meringue atop a rich mouth-puckering filling. A few tricks are in order, however, to help prevent common mishaps, such as a soggy crust or runny filling. First, the crust must be fully blind-baked so that it gets crisp and firm. A half-butter, half-shortening crust like the one used here has a pleasantly crumbly, tender texture. (Substitute an all-butter crust, if you prefer.) The stove-top custard filling is thickened with egg yolks and cornstarch. It must come to a full boil and then cook for several minutes in order to activate the cornstarch and thicken properly.