To crack the spices, place them in two zipper-lock bags, one inside the other, and press or gently pound with a skillet, rolling pin, or meat mallet. We prefer Demerara and turbinado sugar for their crunch, but plain brown sugar works too.
PREPARE GRILL: Heat all burners on high for 15 minutes. Leave primary burner on high and turn other burners off. (For charcoal grill, open bottom vent on grill. Light about 100 coals; when they are covered with fine gray ash, spread over half of grill. Set cooking grate in place and heat covered, with lid vent open completely, for 5 minutes.) Scrape and oil cooking grate.
COAT PORK: Meanwhile, pat pork dry with paper towels. Combine mustard seeds, coriander seeds, peppercorns, sugar, salt, and cornmeal on rimmed baking sheet. Place cornstarch in large bowl. Beat egg whites in second large bowl until foamy. One at a time, coat tenderloins lightly with cornstarch, dip in egg whites, and transfer to rimmed baking sheet to coat with spice mixture.
GRILL PORK: Spray tenderloins lightly with cooking spray and grill, covered, over hot side of grill, turning occasionally, until browned all over, 6 to 8 minutes. Slide pork to cooler side of grill and continue to cook, covered, until meat registers 145-degrees, 6 to 12 minutes longer. Transfer pork to carving board, tent with foil, and let rest 5 minutes. Slice and serve.