Place onion, apple juice, water, and liquid smoke in inner steel pot of pressure cooker.  Season ribs with kosher salt and pepper. Place ribs in pressure cooker.  Lock pressure cooker lid in place and set steam vent handle to Sealing. Select Pressure Cook and cook on High Pressure for 20 minutes. Let pressure release naturally for 15 minutes, then turn steam vent handle to Venting to quick-release remaining pressure.  Preheat your oven’s broiler. Line a baking sheet with aluminum foil.  Transfer rib racks to prepared baking sheet and brush with barbeque sauce.  Place ribs under preheated broiler until sauce caramelizes, 3 to 5 minutes.