1. Combine all ingredients and mix. Ladle mojo to cover Mahi, reserving remaining mojo. Marinate for two hours.
2. Sauté garlic, onion and chili pepper until soft. Add beans and stock. Cook 15 minutes. Place in blender, process until smooth, Season.
3. Heat salad oil in sauté pan until very hot. Place fish and cook approximately 2 minutes on each side. Ladle black bean sauce on serving plate and set fillet on top. Ladle a little of remaining mojo on fish. Serve over mash potatoes, if desired. Garnish with grapefruit sections.