Preparation

• Position a rack in the center of the oven and heat the oven to 400°F.
• Lightly brush both sides of the tortillas with oil and arrange them in a single layer on two large heavy-duty baking sheets. Lightly season the tortillas with salt and bake, swapping and rotating the pans halfway through, until golden-brown and crisp, about 15 minutes.
• Meanwhile, in a small bowl, stir the sour cream, cilantro, lime juice, and 1/4 tsp. salt. Cover with plastic wrap and refrigerate.
• Heat the olive oil in a 3-quart saucepan over medium heat. Add the onion and cumin seeds and cook, stirring, until the onion is tender and golden-brown, about 8 minutes. Add the beans and chile and stir until heated through. Stir in 2 Tbs. of the reserved pan juice and 1/4 cup water; bring to a simmer. Using a potato masher, coarsely mash the bean mixture and then season to taste with salt.
• Combine the chicken and the remaining 2 Tbs. pan juice in another 3-quart saucepan over low heat. Cover and cook, stirring often, until the chicken is hot, about 2 minutes.
• Arrange 2 tortillas on each of 4 large plates. Spread the bean mixture over the tortillas. Top with the chicken, and then the radishes and lettuce. Spoon some of the cream over the tostadas, sprinkle each with cheese, and serve.

Serve with your favorite salsa.