In a large enameled cast-iron casseroleover medium heat, sauté sausage in butter until it begins to brown, about 4 minutes. Add onion, celery, and bell pepper; cook until softened. Add garlic, tomatoes, red pepper flakes, bay leaves, thyme, and salt. Deglaze pan with white wine, then add rice and stock.
Cover pot and bring mixture to a boil. Reduce heat to low and simmer mixture for 3 minutes. Remove lid, gently stir rice, and add clams. Replace lid and cook another 5 minutes. Then, add mussels, cover again, and cook 5 minutes more.
Check rice (it should now be al dente) and scatter shrimp over top, sprinkle with salt, and replace lid. Cook another 5 minutes, then check shrimp. If they aren’t cooked through, stir them to bottom of pot and let stand 5 more minutes. Stir in lemon juice and serve.