• Preheat broiler.
  • Cut half of onion into 3/4-inch wedges, then chop remainder. Toss onion wedges, tomatoes, garlic, and whole chiles with 2 tablespoons oil in a 4-sided sheet pan, spreading in 1 layer.
  • Broil about 4 inches from heat until softened and charred, 10 to 15 minutes.
  • Meanwhile, toss together lettuce, radishes, chopped onion, and half of cilantro.
  • Purée tomato mixture in a blender along with one or both roasted serranos (to taste) and 1 teaspoon salt until smooth(use caution when blending hot foods).
  • Transfer purée to a bowl and stir in chicken, remaining cilantro, and salt to taste.
  • Heat remaining 1/2 cup oil in a heavy medium skillet over medium-high heat until it shimmers.
  • Fry tortillas, 1 at a time, turning once or twice and pressing with tongs to immerse, until golden brown, 45 to 60 seconds per tortilla.
  • Drain briefly on paper towels, then transfer to plates.
  • Spread tortillas thickly with heated refried beans, then top with chicken mixture.
  • Slice avocado over tostadas and dollop with cream.
  • Mound lettuce mixture on top and sprinkle with cheese.
  • Serve with: Margaritas!!