Seafood Paella

In a medium saucepan, boil 3 1/2 cups of salted water. Add the shrimp shells and simmer, covered, for about 10 min. Strain the broth, and return it to the saucepan. Toast the saffron gently (in a dry skillet), crush the threads with the back of a spoon, and add to the shrimp shell broth. Taste for salt; the broth should be well seasoned.

In a 14-inch paella pan, heat the oil on high. Meanwhile, pat the shrimp and scallops dry. When the oil is hot, sauté the shrimp and scallops until almost cooked through, about 2 min. Transfer to a plate and set aside. Pour out all but 1 Tbs. of oil from the pan. Reduce the heat to medium and sauté the onion and garlic until the onion softens, about 5 min. Add the tomato, season with salt, and sauté until the mixture, called the sofrito, has darkened and is a thick purée, 10 to 15 min.

Meanwhile, bring the shrimp shell broth back to a simmer. When the tomato-onion sofrito is ready, add the rice to the pan. Sauté until the rice loses its opaqueness, about 1 min. Increase the heat to medium-high. Pour in 3 cups of the simmering broth (reserving the remaining 1/2 cup) and stir or shake the pan to evenly distribute the rice in the pan. As the liquid comes to a boil, arrange the mussels or clams in the pan, submerging them as much as possible below the level of the liquid. From this point on, do not stir the rice.

Cook the paella on medium-high, rotating and moving the pan over one or two burners to distribute the heat. When the rice begins to appear above the liquid, after 8 to 10 min., reduce the heat to medium low. Continue to simmer, rotating the pan as necessary, until the liquid has been absorbed, about 10 min. more. Taste a grain of rice just below the top layer; it should be al dente. (If the rice is not done but all the liquid has been absorbed, sprinkle a bit of hot broth to the pan and cook a few minutes more.) Arrange the shrimp and scallops in the pan.

Cover the pan with foil and cook gently for another 2 min. to help ensure that the top layer of rice is evenly cooked. With the foil still in place, increase the heat to medium-high and, rotating the pan, cook for about 2 min., until the bottom layer of rice starts to caramelize, creating the socarrat. The rice may crackle somewhat, but if it starts burning, remove the pan from the heat immediately.