1. Heat 2 tbsp of the oil in a paella pan or a large frying pan over medium heat. Add the shrimp and cook about 2 minutes. Transfer to a plate. Add the langoustines to the pan, cover, and cook for 5 minutes, until the shells are red. Add to the shrimp. Add 1 tbsp oil to the pan, heat, and add the squid. Cook 1 minute, until barely opaque, and add to the plate. Add the mussels to the pan with ¼ cup water. Cover and cook about 4 minutes, or until they open. Transfer mussels and their juices to the plate.

2. Heat 2 tbsp oil in the pan. Add the tomatoes and saffron and cook for 1 minute. Stir in the rice, then the stock.

3. Simmer, uncovered, over medium-low heat for 15 minutes. Season with salt and pepper to taste. Nestle the shrimp, langoustines, squid, and mussels in the rice and let cook until the rice is just tender.

4. Remove from the heat and tent with aluminum foil. Let stand for 5 minutes. Garnish with the lemon and parsley and serve hot.