To the blender, add all of the ingredients for the achiote marinade. Blend on high until smooth. Taste for salt and set aside.
Season the pork ribs with salt and pepper on both sides. Set aside.
Line the bottom of roasting pan with foil paper. Add the sliced onion rings to the bottom of pan. Layer the ribs across in an even layer.
Pour in 1 cup of water to the bottom of pan. Now pour the marinade over the top of pork to coat evenly. Cover tightly with foil paper and roast in a preheated 350 degree oven for 3 hours.
After 3 hours, uncover ribs and baste with the sauce from the pan. Place back in the oven and roast uncovered for 30 minutes. Baste one more time and back in the oven for another 30 minutes. Remove from oven and let the ribs rest for 15 minutes before serving. Serve with a fresh salad, rice and corn.