Bring a cup of water to boil in a small saucepan. Cool slightly. Place dried chiles in a small prep bowl and pour the boiled water over them. Allow chiles to soak and soften, about 10 minutes. Remove from water and remove the stem and seeds (you can leave in a few seeds for added spice or heat as desired).
In the bowl of a food processor, add tomatoes, chopped onion, garlic cloves, and the soaked chiles. Season with a pinch of kosher salt and black peppercorns. Pulse the mixture to create a tomato sauce. Purée until smooth.
Using a medium bowl fitted with a mesh strainer, strain the tomato sauce until smooth. Discard the remaining purée. Set to the side until ready for use.
In a large dutch oven, heat 4 tablespoons of olive oil over medium heat. Add carrots and celery and season with a large pinch of kosher salt and ground black pepper. Cook until softened and translucent, about 3 to 5 minutes.
Add in potatoes, the puréed tomato sauce, fish stock, the dried bay leaf, and 1 tablespoon dried Mexican oregano. Stir everything to combine then bring to a boil. Reduce heat to a simmer to allow the potatoes, carrots, and celery to cook for about 15 minutes until the vegetables are fork tender.
Once vegetables are cooked through, remove the bay leaf and discard. Add in the diced fish chunks and shrimp. Continue to simmer for 5 to 10 minutes more until fish is tender and shrimp has just turned pink. Taste and adjust seasoning as desired.
Ladle soup into bowls. Garnish with cilantro leaves, a wedge of lime, and, if desired, a few dashes of your favorite hot sauce. Serve with a warm pile of fresh tortillas.