For the avocado spread and shrimp:
2 medium avocados, halved and pitted
1 medium scallion, finely chopped
4 teaspoons freshly squeezed lime juice, divided
1/2 teaspoon fine salt, divided
2 tablespoons vegetable oil
1 pound uncooked large shrimp (21 to 25 count), peeled and deveined, tails removed
1/2 teaspoon chili powder
1/2 teaspoon ground cumin
1/4 teaspoon dried oregano
For the tostadas:
4 tostada shells
1 cup shredded iceberg lettuce
2 radishes, thinly sliced
Pickled red onions
Coarsely chopped fresh cilantro
Mexican hot sauce
For the avocado spread and shrimp: Scoop the avocado flesh into a medium bowl. Add the scallion, 2 teaspoons of the lime juice, and 1/4 teaspoon salt. Stir and mash with a fork into a chunky paste; set aside.
Heat the oil in a large frying pan over medium-high heat until shimmering. While the oil is heating, pat the shrimp dry with paper towels and place in a large bowl. Add the spices and remaining 1/4 teaspoon salt and toss to combine. Place the shrimp in an even layer in the pan and cook, stirring rarely, until pink, opaque, and just cooked through, about 3 minutes.
Remove the pan from the heat, add the remaining 2 teaspoons lime juice, and stir to combine; set aside.
Assemble the tostadas: Place the tostadas on a work surface. Divide the avocado spread over the tostadas and spread into an even layer. Divide the lettuce over the avocado spread, then top with the shrimp and radishes. Top with pickled onions and cilantro. Serve with lime wedges and hot sauce.