Heat the oil in a medium (5- to 6-quart) soup pot over medium. Add the chile and stir-fry until they have changed color slightly and are very toasty - fragrant, 30 seconds to a minute. (Don't over toast the chiles or the soup will be bitter.) Scoop up the chile pieces with a slotted spoon, pressing them against the side of the pot to leave behind as much oil as possible, and transfer to a blender jar; set the pan aside. (A food processor will work, but it won't completely puree the chile.) Pour the tomatoes, with their juice, into the blender.
Add 2/3 of the onions and all the garlic to the hot pan. Cook over medium, stirring frequently, until golden, about 7 minutes. Meanwhile, scoop the remaining onion into a strainer rinse under cold water; and set aside to use as a garnish. Use your slotted spoon to transfer the onions and garlic to the blender, and process until smooth.
Set a medium-mesh strainer over the pot and work the tomato-chile mixture through it. Return the pot to medium-high heat and cook, stirring frequently, until reduce and thick, about 6 minutes. Add the broth, potatoes and epazote (if you have it). When the mixture comes to a boil, reduce the heat to medium-low and simmer until the potatoes are tender, usually about 15 minutes. Taste and season with salt, usually a generous 1 1/2 teaspoons.
Just before serving, raise the heat to medium-high and add the mussels or clams and fish. Boil briskly until the bivalves have opened, usually about 4 minutes.
Ladle into large bowls. Sprinkle generously with cilantro and the remaining onion. Serve your steaming bowls of beauty with the limes passed separately for each person to squeeze inal gusto.
Riffs on the Seafood Soup Theme: The fish and shellfish can easily change to include practically any seafood you can lay your hands on. Here are my general guidelines: Mussels or clams add complexity to the soup, so I always try to include them. Shrimp and scallops are good for meaty sweetness. Most medium-or large-flake fish can be used in this dish. (Fish with a fine flake, like sole or small flounder, tend to fall apart in the soup, and strong flavored fish, such as mackerel, bluefish and salmon, can overwhelm the flavors of the broth.) If you want to use crab meat, add it just before you serve the soup so it doesn't disintegrate.
The potatoes can be replaced by cubes of chayote (I don't even peel them), 1-inch lengths of green beans, peas or corn (use the same weight). Or replace them with a drained 28-ounce can of hominy to make a dish similar to the pozole de mariscos that's popular on Mexico's west coast.