Grab a large baking dish that your rack of ribs will fit in. Pour the beer in then sprinkle the sliced onion throughout the pan.
Place the rack of ribs in the pan, bone side down.
Cover the ribs with aluminum foil and bake for 2 hours.
Once the ribs have finished baking, preheat your grill to medium-low. Brush one side of your ribs with BBQ sauce and grill them sauce side down for 5 minutes or until charred to your liking. Add sauce to the other side, then flip and grill the other side for 5 minutes.