In a small bowl combine cumin, paprika, chili powder, garlic, onion, salt, pepper and brown sugar. Mix well and set aside.
Preheat your oven to 275 degrees.
Apply the spice mixture all over your beef rib racks, top and bottom. Use all of it and massage the mixture into the ribs.
Place each rack on top of each piece of tinfoil and wrap up your ribs.
Place the ribs with the bony side up or the meaty side down and fold the top and sides over so the ribs are totally sealed inside.
Do not pile your ribs on top of each other. Put the ribs into the oven, on the middle rack, for between 3 and 4 hours.
After 3 or 4 hours, take the ribs out of the oven open up one pouch. Be careful, hot steam will pour out of the tinfoil when you open it.
We find that our ribs are finished at just over 3 ¼ hours. The ribs are done when you can see more of the bone then when you put it in. The meat will shrink. Using a fork check the meat to make sure it is fall off the bone tender. If they are not ready just reseal the pouch and bake for a little longer. I would check again after 15 minutes.
Once ribs are cooked, remove from oven and set oven to broil.