1. In a bowl, mix sugar and cornstarch. Add raspberries and rhubarb and gently mix to coat.
2. Scrape mixture into unbaked bottom crust in pan. Cover with lattice top and flute edges (see notes). Line a 12-inch pizza pan or a 10- by 15-inch baking pan with foil (to catch drips) and set pie on foil.
3. Bake on bottom rack of a 375° regular oven or 350° convection oven for 30 minutes. Meanwhile, in a small bowl, beat egg yolk with 2 teaspoons water to blend. Brush lattice top and crust edges lightly with yolk mixture (save extra for other uses or discard). Continue baking until filling bubbles in the center, 40 to 50 minutes longer in regular oven, about 1 hour and 40 minutes in convection. If crust browns too quickly (check 10 minutes after brushing with egg yolk mixture), cover pie loosely with foil.
4. Let pie cool on a rack at least 2 hours. Serve, or chill airtight up to 1 day.