Prepare the Saffron Stock: In a saucepan, warm the fish stock over medium heat. Add the saffron threads, allowing them to steep and infuse the stock with their color and flavor.
Sauté the Vegetables: In a large paella pan or wide shallow skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent. Stir in the garlic and cook for an additional minute.
Cook the Bell Peppers: Add the red and yellow bell peppers to the pan, cooking until they begin to soften.
Add the Tomato: Incorporate the diced tomato, cooking until the mixture becomes saucy.
Incorporate Rice: Sprinkle the paella rice evenly across the pan, stirring to coat the grains in the vegetable mixture.
Pour in Saffron Stock: Slowly pour the warm saffron-infused fish stock over the rice and vegetables. Season with salt and pepper. Stir gently to combine, then spread the rice out evenly. Reduce the heat to a simmer and cook without stirring, allowing the rice to absorb the liquid.
Add the Seafood: When the rice is halfway done, about 15 minutes in, arrange the squid, shrimp, mussels, and clams on top of the rice, pressing them down slightly.
Cook Until Done: Continue cooking until the rice is tender, the liquid has been absorbed, and the seafood is cooked through, about 25 minutes. The ideal paella has a slightly toasted bottom layer of rice known as 'socarrat'.
Rest and Serve: Remove the pan from the heat and cover with a clean kitchen towel. Let it rest for 5 minutes. Garnish with lemon wedges and freshly chopped parsley before serving.