Combine oxtail and water in soup kettle. Bring to a boil, reduce heat, and simmer, covered, 2 hours. Remove tail, chop meat, and discard bones. Cool stock, chill, and skim fat from top. Combine meat and stock in clean kettle and simmer 5 minutes. Melt butter in skillet and sauté green peppers, onions, and garlic until soft. Sprinkle with flour and stir until blended. Add several cups hot stock from kettle and stir until smooth. Add sautéed vegetables mixture to kettle and simmer 20 minutes. Garnish with sour cream.