Chicken Tostadas

Name: Chicken Tostadas
Ingredients: 1 medium white onion; 1 pound tomatoes quartered; 2 large garlic cloves; 1 or 2 fresh serrano chiles; 1/2 cup plus 2 tablespoon vegetable oil; 3 cups shredded iceberg lettuce; 6 large radishes halved and sliced; 1/2 cup chopped cilantro; 1 rotisserie chicken meat coarsely shredded (4 cups); 6 (6-inch) corn tortillas; 1 (1-pound) can refried beans; 1 avocado, halved pitted; 1/2 cup mexican crema or sour cream; 1/4 cup crumbled queso añ; Accompaniments: sliced serranos