Preparation

Make the pork

Heat a medium skillet over medium-high heat. When hot, add the anchos to the skillet, and cook until fragrant and lightly toasted, 20 to 30 seconds per side. Transfer the anchos to a bowl, and pour in enough hot water to cover. Let stand until soft and pliable, 10 to 15 minutes.

Meanwhile, add the onion to the skillet, and cook, stirring occasionally, until lightly charred, about 4 minutes. Transfer the onion to a blender along with the anchos, 1/2 cup of the soaking liquid, the sesame seeds, raisins, tomatoes, garlic, chocolate, cumin, and cinnamon; purée until smooth, scraping down the blender if necessary.

Season the pork all over with 1 tsp. salt and 1/2 tsp. black pepper. Put the pork in a 6-qt. slow cooker and pour the ancho mixture over it. Cover and cook on low until very tender, about 6 hours. Shred the pork with two forks, season to taste with salt and black pepper, and continue cooking on low, uncovered and stirring occasionally, until the liquid thickens slightly, 15 to 20 minutes.

Make the relish and assemble the tostadas

Combine the jícama, radishes, jalapeño, cilantro, lime juice, sugar, 1 tsp. salt, and 1/4 tsp. black pepper in a bowl; toss. Season to taste with salt, black pepper, and lime juice.

To serve, place 2 tostada shells onto each of 8 plates. Top each with 1/2 cup pork and 1/4 cup relish. Drizzle each serving with some of the crema. Garnish with sliced radish and jalapeño, and cilantro, if you like.