Step 1

Preheat oven to 425 ° F. Lightly grease an 8-inch-square broiler-safe baking dish. Puree salsa, sour cream, and cilantro in a blender until smooth, 20 to 30 seconds. Season with salt and pepper.

Step 2

Spread 1/2 cup sauce on bottom of prepared pan. Transfer remaining sauce to a bowl. Add chicken, scallions, and 1 cup cheese; toss to combine. Season with salt and pepper.

Step 3

Place 4 tortillas on top of sauce in pan, slightly overlapping. Top with one-third of chicken mixture (about 1 1/3 cups) and 4 more tortillas. Repeat two more times. Top with remaining chicken and 1/2 cup cheese.

Step 4

Cover with foil and bake until warmed through, 15 to 20 minutes. Uncover and broil until browned, 1 to 2 minutes. Serve sprinkled with onion and cilantro.