Melt 8 ounces (2 sticks) butter in a heavy saucepan. Add the flour and cook, stirring frequently, over medium heat until the roux is light brown. Set aside.
In a 5-quart saucepan, melt the remaining butter and add turtle meat. Cook over high heat until the meat is brown. Add celery, onions, garlic and seasonings, and cook until the vegetables are transparent.
Add tomato purée, lower heat and simmer for 10 minutes. Add the stock and simmer for 30 minutes. Add the roux and cook over low heat, stirring, until the soup is smooth and thickened. Correct seasoning with salt and pepper to taste. Add lemon juice, eggs and parsley.
Remove from heat and serve. At the table, add 1 teaspoon sherry to each soup plate.