Heat small pot of water to boiling. Meanwhile, remove thorns from cactus leaf by scraping with knife, then chop. Add to boiling water and boil for 15 minutes. Drain and set aside.
Heat grease in large skillet over medium heat. Add catfish in single layer, then add garlic and onion and cook until fish is white all the way through, flipping halfway (5-10 minutes). Remove from pan and set aside.
Add mushrooms to pan and saute for about 7 minutes, until almost done to your liking. Add spinach and cactus, blending well and return catfish to pan. Turn off heat and let the residual heat finish cooking the spinach.