Wash eel, skin it, remove backbone and cut into bite-sized pieces. Soak mushrooms in boiling water for 30 minutes then remove and discard stems. Cut capsicum into 2½ cm squares. Drain pineapple, reserving ½ cup of the canning juice.
Heat 1 tablespoon oil in pan over medium heat and fry garlic and onions until soft. Add eel, capsicum, fish sauce and sprinkle of pepper to taste. Stir-fry until eel has softened, adding more oil if necessary. Add pineapple pieces, tomatoes and mushrooms, stirring to heat through.