Black Bean Tostadas With Sweet Potato and Poblanos

Name: Black Bean Tostadas With Sweet Potato and Poblanos
Ingredients: 1 sweet potato, chopped; 3 poblano peppers, seeded and chopped; 1 teaspoon ground cumin; 4 tablespoons olive oil, divided; ¾ teaspoon kosher salt, divided; 2 tablespoons fresh lime juice (from 1 lime); 3 cups shredded red cabbage (from 1 small head); 4 radishes, thinly sliced; 1 cup chopped fresh cilantro leaves; 1 15.5-oz. can refried black beans, warmed; 4 5-in. corn tostadas; ¼ cup sour cream; Lime wedges, for serving