Place a medium-size frying pan over medium heat and pour in half of the vegetable oil. Add the fish and sear for a few seconds, just enough to seal it on both sides. This step will give the fish a better flavor, as well as keep it from falling apart when it’s in the soup. After you’ve seared the fish, set aside.
Roast the tomato, garlic, and onion in the same frying pan until the skin of the tomato (and only that of the tomatis slightly charred. Afterward, set all these ingredients aside.
In the same pan, fry the tortilla until lightly golden (but be careful not to burn it). The tortilla will add a thick consistency to the broth.
Using your blender, blend the tomato, onion, garlic, and jalapeño pepper with a little bit of water (you can use fish or seafood broth if you have it). Add salt and black pepper to your liking, along with the fried tortilla, and blend again.
Once all the ingredients are mixed and you have a fine sauce, fry it with a little bit of vegetable oil in a pot at medium-high heat. Cook for about three minutes or until the sauce acquires a darker color. It is at this moment that you add the seafood. Remember to start with the seafood with the thickest shells, like the crawfish (or prawns, if usinand the crab.
Continue cooking for about 10 minutes at medium-low heat so that these ingredients can release their characteristic seafood flavor. You will be able to tell that the crawfish and the crab are fully cooked when their color turns from blue to red. It is at this point that you add more water and place a lid on the pot. Let it continue to boil for another 20 minutes.
After the 20 minutes, uncover the pot and add the rest of the ingredients: the fish, shrimp, and epazote. Let simmer for another 10 minutes. Taste again, and make any adjustments to your liking. Serve the soup with fresh corn tortillas, Mexican salsa (pico de gallo), and lemon juice.