In a medium pot, add 1 tablespoon of oil and heat to medium. Add 1/4 of the sliced onion and half of the minced garlic. Saute for 3 minutes. Add the tomatillos and chile negro. Saute for another 3-4 minutes. Pour in enough water to cover ingredients. Bring up to a simmer and cook for 7-8 minutes. Remove from heat and let sit for 10 minutes.
While the salsa ingredients cool, heat 2 tablespoons of oil in a large saute pan to medium heat.
Season the beef ribs lightly with salt and pepper all over. When oil is hot, transfer beef to pan. Sear and brown the ribs, turning as needed.
While beef browns, drain all of the water from the cooked tomatillos and chiles. Transfer to the blender jar. Add 1 3/4 cups water, 2 teaspoons of bouillon, pinch of Mexican oregano, salt and pepper to taste. Blend on high until very smooth. Taste for salt. Set aside.
Once beef is browned, add the reserved sliced onion and saute for 2-3 minutes. Add the garlic and saute for one more minute.
Add the previously cooked cactus and the salsa from the blender. Stir well to combine. Reduce the heat to a light simmer. Cover partially and continue cooking for 35-40 minutes or until beef is very tender. Serve with rice, beans and flour tortillas.