In a bowl or shallow dish, combine the first 6 ingredients. Add the halibut and turn to coat. Cover and refrigerate for 30 minutes. In a large cast-iron or other heavy skillet, stir-fry red peppers and onion in 1 tablespoon oil until tender, about 5 minutes. Remove from pan; set aside. Add 2 tablespoons oil to pan; add halibut. Cook and stir until fish just begins to flake easily with fork, 5-8 minutes. Stir in the milk, tomato paste, mangoes and reserved pepper mixture. Cook over medium heat for 5 minutes.
Meanwhile, in a Dutch oven, cook and stir spinach in remaining 2 tablespoons oil until wilted, about 2 minutes. Serve fish over spinach.
