Preheat the oven to 300°F.

Combine the first 6 ingredients to make the dry rub spice blend, and rub the spice blend all over the baby back ribs. Place two large sheets of heavy-duty aluminum foil (or double up sheets of regular foil) on the counter. Divide the onions between the two rectangles of foil and lay the ribs on top of the onions. Tightly close the foil around the ribs so that they are steam-tight.

Place the two foil packets on a sheet pan and bake in the oven for 2½ hours for fork tender ribs or 3 hours for falling off the bone tender ribs.

While the ribs are cooking, start the BBQ sauce. Combine the ketchup, molasses, cider vinegar, tomato paste and soy sauce in a small saucepan and simmer.

When the ribs are finished cooking, remove them from the oven and let them sit for 5 minutes. Carefully open the foil packets and transfer the ribs back to the sheet tray. Pour the cooking juices from the foil packet into the saucepan with the BBQ sauce. Bring the sauce to a boil, lower the heat and simmer for 5 to 10 minutes until thickened.

Pre-heat the broiler. Brush both sides of the ribs with the thickened BBQ sauce and broil the ribs for 5 to 10 minutes, until the sauce is bubbling on the ribs.

Cut the ribs unto 3-rib sections and brush with more BBQ sauce before serving.