1. Rinse tomatillos. Place tomatillos and poblanos in a 10- by 15-inch rimmed pan. Broil 4 to 6 inches from heat, turning as needed, until tomatillos and poblanos are blackened all over, about 20 minutes total. Set aside as charred. When cool enough to handle, pull off and discard poblano peels, stems, seeds, and veins.
2. In a blender or food processor, whirl (in batches if necessary) tomatillos, poblanos, pumpkin seeds, garlic, cilantro, and broth until chili sauce is smoothly puréed. Add serranos to taste and whirl to purée.
3. Meanwhile, peel onion and cut 3 or 4 thin slices from the center. Separate slices into rings and wrap airtight. Finely chop remaining onion.
4. Spread 1 cup chili sauce over the bottom of a shallow 3-quart casserole (about 10 by 12 in.). Cover sauce with 1/3 of the tortillas, overlapping them. Scatter 1/2 the chicken, 1/2 the chopped red onion, and 1 cup cheese over tortillas. Spoon 1 cup chili sauce over cheese. Cover completely with another 1/3 of the tortillas. Scatter remaining chicken, remaining onion, and 1 cup cheese over tortillas; moisten evenly with 1 cup chili sauce. Cover with remaining tortillas, then coat tortillas evenly with remaining sauce.
5. Bake in a 400° oven until sauce is bubbling at edges of casserole, about 15 minutes. Sprinkle remaining cheese on enchiladas and bake until it melts, 2 to 3 minutes.
6. Scatter red onion rings over enchiladas and garnish with cilantro sprigs. Cut into rectangles and serve with a wide spatula. Add salt and sour cream to taste.