Season your ribs with Garlic powder, salt, and pepper (if desired) Make sure to season both sides and rub it in.
Preheat the oven to 250 degrees. Arrange your sliced onions on the bottom of the baking dish. Top with your ribs, and cover the bottom of the pan with water (don't fully cover the ribs with water) Cover with foil.
Bake at 250 for 3 hours. No longer.
To make the BBQ sauce. Combine all ingredients for the sauce in a medium sauce pan. Bring to a boil. Once at a boil the brown sugar should melt and blend in. Turn down to a simmer for 20 minutes. Do this part at the last 45 minutes of the ribs cooking.
Pull the ribs out of the oven when done. Turn the oven up to 350 degrees. Drain the juices from the bottom of the pan (or transfer ribs and onions to another pan) Cover with BBQ sauce (you don't have to use all of it, just enough to make sure all the ribs are covered) bake uncovered for 30 minutes