This casserole is baked in a pie plate and is a simple way to use a classic tamale batter. Although it lacks some of the flourish of individually steamed packets, it's a dish that welcomes a variety of fresh fillings and flavor.
Heat the broiler. Place the tomatillos on a baking sheet. Place the baking sheet 4 inches below the heating element. Broil tomatillos, turning once, until soft and blackened in spots, 8 to 9 minutes. Remove baking sheet from broiler, and set aside until tomatillos are cool. Position the oven rack in the upper third of the oven. Reduce the temperature to 425 degrees. Lightly oil an 8-inch pie plate; set plate aside.
Place garlic in a medium skillet set over medium heat. After 5 minutes, add chiles. Cook, turning occasionally, until chiles and garlic are soft and blackened in spots, about 10 minutes more. Remove skillet from heat; set aside to cool. Peel the garlic.
Transfer the tomatillos, any accumulated juice, chiles, and garlic to the bowl of a food processor or the jar of a blender. Cover, and pulse until a coarse puree forms. Transfer puree to a medium bowl.
Place the onions in a sieve, and rinse under cold running water. Drain, and add to tomatillo mixture along with cilantro and salt; stir salsa until combined.
Measure 3/4 cup salsa into a medium saute pan, and stir in the chicken. Set over medium heat, and cook, stirring, until chicken becomes tender, 8 to 10 minutes. Remove from heat, and set aside to cool.
Spread half of the tamale batter over bottom of prepared pie plate. Spread the chicken filling evenly over the batter, then spread remaining batter over chicken.
Bake until golden, about 5 minutes.
Reduce temperature to 300 degrees, and lightly brush top of pie with oil; loosely cover with foil. Continue baking until golden and center springs back when touched, about 35 minutes more. Remove casserole from oven, and let stand 5 minutes.
Cut the casserole into wedges. Serve casserole warm, and drizzle with the remaining tomatillo salsa; if salsa is too thick to drizzle, thin with 1 or 2 tablespoons water.