Make the tomato seasoning. Heat your broiler. On a baking sheet, lay out the onion slices, unpeeled garlic and chiles (if using jalapeños, cut them in half lengthwise). Roast them about 4 inches below the hot broiler for 5 or 6 minutes, until they are blistered and blackened in spots, then flip them over and roast the other side. Scrape them into a food processor or blender, add the tomatoes (juice and all) and process to a coarse puree.
Finish the dish. In a large (4-quart) saucepan, heat the oil over medium-high. When hot, add the tomato seasoning and stir until the mixture is thick and shiny, about 5 minutes. Add the rice and stir for another minute or so, then add the broth and bring to a rolling boil. Reduce the heat so that the broth very gently simmers and add the fish, mussels (or clams) and epazote (if you have it). Cover the pot and set the timer for 15 minutes. When the timer goes off, taste a kernel of rice: If it’s still crunchy inside, re-cover the pot and cook a couple of minutes longer. If it’s close to done, add the shrimp, re-cover the pot, turn off the heat and let the shrimp cook gently in the warm, steamy environment for 2 or 3 minutes. Stir in the cilantro or parsley, then taste and season with salt (this can range from none to 1 ½ teaspoons, depending on the saltiness of the broth). Ladle into bowls and serve with the lime wedges for each person to squeeze on al gusto.