Toast the guajillo peppers and optional chipotle/ancho briefly in a dry skillet over medium heat for 20–30 seconds per side until fragrant but not burnt. Transfer to a bowl and cover with hot water; soak 15 minutes until softened. Reserve 1/4 cup of soaking liquid and discard seeds/skins. In a blender, combine the softened peppers, reserved soaking liquid, half of the chopped tomatoes (about 3/4 cup), 1/4 cup chopped onion and 1 clove garlic. Blend to a smooth purée and set aside.

Make the soup base

In a large heavy pot or Dutch oven, heat 2 tablespoons olive oil over medium heat. Add the remaining chopped onion and sauté 4–5 minutes until translucent. Add the minced garlic, diced carrot, and diced celery; cook 3–4 minutes until aromatic and beginning to soften. Stir in diced potato and chopped remaining tomatoes; cook 2–3 minutes, breaking tomatoes with the back of a spoon. Add the puréed chiles, ground cumin, dried oregano, and optional sugar; stir to combine.

Simmer with stock

Pour in 8 cups fish or seafood stock and 2 cups water. Add the bay leaf and bring the pot to a gentle boil, then reduce heat to a simmer. Simmer uncovered 15–20 minutes, or until potatoes and carrots are tender and flavors have melded. Taste and season with 1 to 1 1/2 teaspoons salt and 1/2 teaspoon black pepper, adjusting as needed.

Add seafood

Turn the heat to medium-low. Add the firm white fish pieces first and simmer gently 3 minutes. Add the shrimp and scallops and simmer 2–3 minutes more until shrimp turn pink and scallops are opaque. Finally add the mussels or clams, cover the pot, and cook 3–5 minutes until shells open (discard any that do not open). Do not overcook—seafood should be just cooked through.

Finish and garnish

Turn off heat. Stir in 2 tablespoons fresh lime juice, chopped cilantro, and fresh oregano if using. Taste and adjust salt, pepper, or lime. Ladle the caldo into bowls and serve with lime wedges, diced avocado, and sliced radishes on the side for garnish. Offer warm corn or flour tortillas or crusty bread if desired.

Make-ahead and storage

If preparing ahead, make the base (through simmer with stock) and cool; refrigerate up to 2 days. Add seafood and finish just before serving to avoid overcooking. Leftovers with seafood can be refrigerated 1–2 days; longer storage is not recommended for cooked shellfish.