In a large skillet, heat the oil, add the shrimp and cook for 30 seconds. Add the onions, garlic, 1/2 teaspoon salt, and the coriander. Cook until the onions are tender, about 3 minutes. Add the tomatoes, orange zest, broth and 1 tablespoon of cornmeal. Bring to a boil, stirring, until the mixture thickens, about 2 minutes. Remove, stir in the fresh tarragon and set aside. Preheat oven to 350 degrees. In a large saucepan, whisk together 4 cups of water and the remaining cornmeal and salt. Bring to a boil over medium heat, whisking frequently, until the mixture boils. Reduce the heat to low and cook ,stirring constantly until thick. Remove from the heat and stir in the butter. Put 1/3 of this mixture in the bottom of a buttered 8 by 11-inch casserole dish. Spoon the filling over the cornmeal, spreading it out evenly. Top with the rest of the cornmeal mixture, spreading evenly. Sprinkle on the cheese, bake for 30-35 minutes. Remove from the oven and spread the sour cream topping over the entire dish.
Yields 6 portions