NOTE: If you plan on making the seafood or fish broth, get the whole fish and ask your fish monger to clean it for you and to give you the head, bones and tail to use for the broth. Also, save the shrimp shells and tails to use in the broth, as well.
Cut the fish fillets so that you have 6 more or less equally sized pieces. Season with ¼ teaspoon salt and freshly ground black pepper to taste. Set aside.
Place the tomatoes, jalapeños, and the 3 whole garlic cloves in a medium saucepan, cover with water and bring to a simmer over medium-high heat. Simmer until the tomatoes are fully cooked and very soft, about 10 minutes. Transfer the tomatoes, garlic cloves, and only 1 of the jalapeños (puree one chile at a time, taste for heat, and add the other if desired) to a blender, and add the onion and 1 teaspoon of the salt. Puree until completely smooth.
Rinse and dry the saucepan and heat 2 tablespoons of the olive oil in it over medium heat. Once hot, add the tomato puree and cover the pan partially with a lid, as the puree will sputter and jump. Cook, stirring occasionally, until thick, dark and fragrant, about 6 to 7 minutes. Remove from the heat and set aside.
In a large, wide casserole, heat 1/4 cup of the olive oil over high heat. Once hot, toss in the squid, sprinkle with 1/2 teaspoon of salt, add half of the finely chopped garlic, and cook for 2 minutes, stirring halfway through. Remove the squid and the garlic with a slotted spoon and place in a heatproof bowl.
Add the shrimp to the casserole, along with another 1/2 teaspoon of salt and half of the remaining chopped garlic, and cook for 2 minutes, stirring and flipping the shrimp over halfway through. Remove with a slotted spoon and place in the bowl with the squid.
Reduce the heat to medium and add 1/4 cup of the remaining olive oil to the casserole. Once hot, add the rice and cook for about 3 to 4 minutes, stirring often and scraping the bottom of the casserole, until the rice is crackling and coated with oil, feels heavier in the pan as you stir it, and the color of the grains has changed from a pale white to a deep milky white.
Pour the cooked-down tomato puree over the rice; it will sizzle and smoke a bit, which is what you want. Cover partially with a lid and cook, stirring a couple of times, until the rice absorbs most of the sauce, about 2 to 3 minutes. Add the seafood broth and stir the rice, scraping the bottom of the casserole. Add the epazote or cilantro sprigs and reduce the heat to medium-low. Gently arrange the reserved shrimp and squid on top of the rice, adding any of their juices from the bowl, as well as the clams, mussels and seasoned fish fillets.
Cover and simmer over medium-low heat for 10 to 12 minutes, until the fish is cooked through and can be easily pulled apart with a fork, and the clams and mussels have opened up. Turn off the heat and serve immediately in soup plates. The rice should be tender and the mixture very soupy.