Gently pick through the jumbo lump crabmeat to remove any cartilage or shells. If shrimp are large, chop into bite-size pieces. Place crab and shrimp in a bowl, add 2 tablespoons fresh lime juice, 1 teaspoon kosher salt and 1 teaspoon black pepper. Toss gently to combine, cover, and refrigerate while you make the sauce so flavors meld (10–15 minutes).

Spicy tomato sauce

Cook aromatics and tomatoes

Heat 1 tablespoon oil in a medium skillet over medium heat. Add minced garlic and cook, stirring, about 30 seconds until fragrant. Add the diced tomatoes and 1 tablespoon tomato paste. Stir to incorporate and cook 4–6 minutes until tomatoes begin to break down.

Season and simmer

Stir in 1 teaspoon smoked paprika, 1/2 teaspoon ground cumin, 1 teaspoon chili flakes (or to taste), 1 teaspoon vinegar, 1/2 teaspoon sugar, 2 tablespoons water, and 3/4 teaspoon salt. Reduce heat to low and simmer 6–8 minutes, stirring occasionally, until sauce thickens slightly. Taste and adjust salt, acid, or heat as needed. If a smoother sauce is preferred, cool slightly and pulse a few times in a blender or use an immersion blender to desired consistency; return to skillet and warm through.

Cool slightly

Remove sauce from heat and let cool 5 minutes so it won’t cook the seafood when combined, but remain warm enough to meld flavors.

Combine and finish

Fold sauce into the chilled seafood: gently mix the seafood (crab and shrimp) with the warm-but-not-hot tomato sauce until evenly coated. Taste and adjust seasoning with more lime juice, salt, or chili flakes as desired.

Add vegetables and herbs: fold in the diced avocado, finely diced red onion, minced jalapeño, and 1 cup chopped cilantro. Stir gently to combine without mashing the avocado. Refrigerate for 10–15 minutes for flavors to marry, but do not leave more than 1 hour to keep avocado fresh.

To serve

Portion and garnish: Spoon the campechana onto tostada shells or serve family-style with tortilla chips. Garnish with an additional 1/4 cup chopped cilantro and lime wedges. Serve immediately.

Make-ahead note: keep sauce and seafood chilled separately up to 1 day; combine and add avocado just before serving to prevent browning.