Heat the stock in a large pot. Shell and devein the shrimp; cover and set aside in the fridge. Place the shrimp shells into the stock and let simmer while you finish the recipe. Clean and prep the vegetables and meat before beginning to assemble the paella.
Preheat oven to 450 F. Add the olive to a traditional paella pan or wide shallow sauté pan over medium-high heat. When hot, add the chicken pieces and brown carefully for about 10 to 12 minutes or until cooked. Add the sausage and sauté about 3 minutes. Then add the minced garlic, bay leaves, onions, green pepper and tomato. Cook and stir occasionally for about five minutes until vegetables soften.
Meanwhile, strain the stock to remove the shrimp shells and add the saffron to the stock and set aside. Add the rice to the softened vegetables and cook, stirring, for about five minutes. Add the stock to the rice along with a few cranks of black pepper and ¼ teaspoon salt. Bring back to a boil. Cover the pan with aluminum foil, transfer to the oven and bake for about 20 minutes.
Add the shrimp, scallops and fish, pressing into the hot rice. Return to the oven and continue cooking until the seafood has cooked through, about 10 minutes. Add the mussels in a ring around the edge of the pan, toss the peas over the top and cover. Put back into the oven for another four to six minutes until the mussels open. (Discard any mussels that do not open after cooking.)
Remove from the oven and let stand, covered, for a few minutes before serving.