Cook's Note: To get a mixture of chipotle peppers and adobo sauce, puree a whole can in a food processor or blender. Keep on hand in your refrigerator to use when you need a bit of smoky heat.
Heat the oil in a skillet pan over medium heat and add the onions. Cook, stirring occasionally, until the onions begin to soften, about 4 minutes. Add the garlic and cook, stirring, until fragrant, about 30 seconds. Add 1/4 of the pico de gallo and the chipotle in adobo, if using, and cook, stirring frequently, until the tomatoes begin to break down, about 4 minutes. Add the chicken broth and bring to a simmer.
Sprinkle the pork with 1 teaspoon salt and 1/2 teaspoon pepper. Add the pork to the skillet with the sauce and return to a simmer. Cover the skillet and simmer, flipping once, until the pork is just cooked through and tender, about 20 minutes. Remove the pork, cover loosely with aluminum foil and set aside for 5 minutes.
Meanwhile, bring the sauce in the skillet to a boil and cook over medium-low heat until slightly thickened, about 5 minutes. Shred the pork with 2 forks, add it back to the skillet and toss well to coat. Season with additional salt and pepper if needed.
Divide the shredded pork among the tostadas. Top with lettuce, cheese and the remaining pico de gallo. Drizzle with crema and top with radish slices. Serve with lime wedges.
Cook's Note: Make your own tostadas by brushing corn tortillas lightly with oil and broiling 6 inches from the heat source, turning once, until golden brown and crisp, 3 to 4 minutes.