Spread the tostadas with the refried beans. Top with the chicken, lettuce, onion rings, radishes, and cheese. Drizzle with sour cream, top with a dollop of salsa, and serve.;
Combine the tomatillos, avocado, cilantro, chile, and lemon juice in a blender and puree until smooth. Season the salsa with salt and pepper, to taste. Transfer to a serving bowl, cover and refrigerate for up to 2 hours or until ready to use.