2) Add 150g of the onions to the pan, and the celery, carrots and garlic, and cook for 5 to 7 minutes. Deglaze with the white wine and reduce by half.
3) Add the seafood stock, saffron and water and leave to simmer for 2 hours, or until reduced by half. When the stock has finished simmering, strain and hold the stock on low heat.
4) In a large saute pan or paella pan, add the remaining 2 tbsp of oil, the chorizo, red pepper and remaining onion. Cook until the onion is translucent, but do not brown. Add the rice, salt and pepper to the pan, and saute until all of the grains of rice are coated with oil.
5) Form the rice around the pan so it is level. Add 240ml of stock at a time to the rice, continuing to add more liquid when each cup is absorbed; do not stir. When the rice is al dente, or three-quarters of the way to being fully cooked, add the chicken thighs, prawns, scallops, mussels and clams. Bury the seafood items in the rice so they will cook.
6) Add the final 240ml of stock and cover the entire pan tightly with another pan or lid. Let the pan sit off the heat for 10 to 15 minutes or until the seafood is cooked. Garnish with spring onions and tomatoes and serve.