Rinse fish thoroughly in cold water; pat dry, and set aside.
Melt shortening in a large heavy skillet. Sauté potatoes until browned. Stir in green pepper, tomatoes, 1 teaspoon salt, and 1/4 teaspoon pepper. Layer fish over vegetables. Cover and cook over high heat 5 minutes. Reduce heat to low; cover and simmer 15 minutes or until fish flakes easily when tested with a fork.
Arrange fish around outside of a serving platter; place vegetables in center of platter. Add salt and pepper to taste. Serve immediately.