Put a large, heavy pot over medium heat (I use a cast iron pan or a Dutch oven). Add the olive oil and slowly cook the diced onion and pepper until tender and browned (about 10 minutes).
Add the rice and stir for a couple of minutes, allowing the rice to coat itself in the oil but making sure it never burns or sticks.
Start adding the fish stock little by little. Cover the rice with about 1/2 an inch of stock and stir every couple of minutes. When it looks dry add more stock. It should take about 20 minutes to add it all.
Add the shellfish after about 15 minutes. They will cook through and open up before the dish is done. If any happen to remain closed, discard.
Start tasting the rice around the 20-minute mark. Adjust for salt now. Depending on the rice used, it usually takes 20-25 minutes to be fully cooked. As soon as it's done, remove it from the heat and let it rest for five minutes before serving.
Enjoy this seafood rice with your favorite Spanish wine!