Indian-Spiced Pork Roast with Rosemary and Onions

Toast peppercorns, fennel seeds, and cumin seeds in a dry small skillet over medium heat, tossing often, until fragrant, about 2 minutes. Let cool, then coarsely grind in spice mill or with mortar and pestle along with red pepper flakes and paprika. Transfer to a small bowl and stir in 3 Tbsp. Diamond Crystal or 1 Tbsp. plus 2 1/4 tsp. Morton kosher salt.
Place racks of pork on a rimmed baking sheet and pat dry. Tie racks with kitchen twine, spacing 1" apart (make sure to tie the meat snugly; you want it to be uniform in shape so that it will cook evenly). Rub all over with spice mixture and chill, uncovered, at least 6 hours or up to 12 hours (for the best flavor, longer is better).
Let pork sit out at room temperature about 2 hours before roasting.
Combine rosemary and 1/2 cup oil in a small bowl; let infuse while meat is coming to room temperature.
Place a rack in middle of oven and preheat to 500°F. Toss onions and bay leaves with remaining 3 Tbsp. oil in a large roasting pan (they should cover the bottom of the pan); season with salt. Rub pork all over with rosemary oil and place on top of onions. Roast until top of pork is golden brown, 10–15 minutes. Reduce oven temperature to 325°F and continue roasting pork and onions until an instant-read thermometer inserted into the center of meat registers 135°F, 60–75 minutes. Transfer pork roasts to a cutting board and tent with foil. Let rest 20–30 minutes. Keep oven on.
Just before serving, give onions a toss and heat in oven just to warm through. Slice pork roasts between rib bones to make individual chops; arrange on a platter with onions.