Combine all the spice rub spices into a bowl and mix. Store in a glass jar with a good lid.
Grind the cumin in a mortar and pestel then add the garlic, salt and chile and crush it all into a paste. Scoop it out into a heat proof bowl. In a small sauce pan heat the oil gently. Once it is a little more then warm pour it over the spices. Add the Sherry vinegar, lime and orange juice. Whisk to combine then store in the fridge.
Dissolve the salt and sugar into the water by whisking it. The idea here is to make sure the ribs are submerged in the brine. So either put the ribs in a plastic food grade bag or into some sort of snug fitting container so when you pour the brine over them they will be submerged. I use a plastic bag and then remove the air. Place the ribs into the brine.
Let the ribs brine in the fridge for two hours. Then remove them from the brine pat them dry. Turn the ribs over so the concave side is up. Using a knife peel back a corner of the white membrane that is attached. Then using a kitchen towel for grip grab hold of the membrane and pull it off.
Rub the spice rub over both sides of the ribs. Wrap the ribs in plastic wrap and put them into the fridge overnight.
Heat the oven to 325° F. Remove the ribs from the fridge and unwrap them. Lay out two large sheets of foil and spread half the sliced onion and half the garlic onto each sheet. Top the onions with the ribs.
Tear off two more sheets of foil of matching size. Place them over the top of the ribs and then crimp the edges of the foil very tightly. You want to seal in all the juices.
Place the foil packets onto a sheet tray with sides and back the ribs for 3 hours or until they are tender but not falling off the bone. A good indication they are done is the meat will have shrunk on the bones. You want some tooth to them. At this point you can either continue to the grill or you can cool the ribs and store them in the fridge right in the foil packets.
Heat your grill. Carefully open the rib packets and remove the ribs off to the side but still on the tray.
Grill the ribs until the exterior begin to crisp and char basting all the while with the juices in the foil packets. Take the ribs back inside and slice them into rib lets then toss them with the warm mojo and cilantro. Serve