In a small bowl, make a paste by mashing together the garlic, soy sauce, sugar, allspice, cayenne, and salt. Rub into the picnic shoulder. Pour the beer into the cooker and add the bay leaves. Set the rack in place. Place half the onions on the rack and set the pork on top.
Spread the remaining onions over the pork. Lock the lid in place and over high heat bring to high pressure. Adjust the heat to maintain high pressure and cook for 55 minutes. Let the pressure drop naturally or use a quick-release method. Remove the lid, tilting it away from you to allow any excess steam to escape. Check the pork for doneness with a meat thermometer inserted into the center. It should register 170°.
If not, lock the lid back in place and return the pork to high pressure for a few more minutes. Remove pork to a platter and set aside in a warm place. Remove the rack. If there is more than 2 cups of sauce, boil vigorously over high heat to reduce.
Whisk in just enough of the cornstarch solution to thicken the gravy while cooking at a low boil for 2 to 3 minutes. Remove the bay leaves, carve the pork, and pour over the gravy. Serves 4 to 6.