1. Rinse tomatillos. Place tomatillos and poblanos in a 10- by 15 inch baking pan. Broil 4 to 6 inches from heat, turning as needed, until blackened all over, about 20 min. total. When cool enough to handle, pull off and discard poblano peels, stems, seeds, and veins.
2. In a blender of food Processor, whirl (in batches if necessary) tomatillos, poblanos, pumpkin seeds, garlic, cilantro, and broth until smooth. Add Serranos and puree.
3. Meanwhile, peel onion and cut 3 or 4 thin slices from the center. Seperate slices into rings and wrap artight. Finely chop remaining onion.
4. Spread 1 cup chili sauce over the bottom of a shallow 3-quart casserole (about 10-12 in). Cover sauce with 1/3 of the tortillas, overlapping. Scatter 1/2 of the chicken, 1/2 of the chopped red onion, and 1 cup of the cheese over the tortillas. Spoon 1 cup chili sauce over cheese. Cover completely with another 1/3 of the tortillas. Scatter remaining chicken, onions 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 F. oven until sauce is bubbling at edges, 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 ciantro sprigs. Cut into rectangles and serve with a wide spatula. Add salt and sour cream to taste.