In a heavy 8-qt. Dutch oven or soup kettle, combine oil and flour until smooth. Cook over medium-high heat for 5 minutes, stirring constantly. Reduce heat to medium. Cook and stir about 5 minutes more or until mixture is reddish-brown (the color of a penny). Add onion and green onions. Cook for 5 minutes, stirring often.
Increase heat to high. Add hot water or broth, tomatoes and corn. Bring to boil. Reduce heat; cover and simmer for 15 minutes. Add shrimp; simmer for 5 minutes or until shrimp turn pink. Season to taste with salt and pepper.