Meanwhile, heat sesame oil in a skillet over medium heat.Saute onion, pineapple, red and green pepper for 1 minute.
Stir in the pineapple juice, sugar, tomato ketchup and salt. Dissolve 4 teaspoons of flour in 2 tablespoon water and make a thick paste.
Pour this mixture into the skillet. keep stirring.
Cook for 2-3 minutes or until sauce is thickened.
Arrange fried fish pieces in a large dish. Pour sweet and sour sauce over the fish. Delicious sweet and Sour Fish is ready to serve.