Shrimp Courtbouillon with Rice

INGREDIENTS

• 1/4 teaspoon cayenne, or to taste
• 5 cups
• 2 1/4 cups water
• 2 1/4 teaspoons salt
• 1/4 cup plus 1 tablespoon all-purpose flour
• 1 cup finely chopped onion (1 medium)
• 2 garlic cloves, minced
• 1 (1-lb) can whole tomatoes in juice
• 2/3 cup long-grain white rice
• 3/4 cup finely chopped green bell pepper (1/2 medium)
• 2 1/2 tablespoons unsalted butter
• 1 1/4 lb medium shrimp in shell (31 to 35 per lb), peeled, reserving shells
• 1 1/2 cups thinly sliced scallion greens (from 2 bunches)