Prepare sauce: Heat water in large pot to boiling.
Add tomatillos, 2 onion halves, 6 garlic cloves, and 7 serrano chiles.
Simmer uncovered 30 minutes.
Let cool slightly, then drain, reserving about 1 cup cooking liquid.
Place solids in blender or food processor.
Add remaining onion half, 4 garlic cloves, 3 chiles, the cilantro, and about 1/2 cup cooking liquid.
Process until smooth.
Heat oil in medium saucepan.
Add onion slices and saute until lightly browned.
Stir in sauce and season with salt.
Simmer uncovered until slightly thickened, about 30- 45 minutes.
If sauce thickens too much, thin it with remaining cooking liquid.
Meanwhile, when the sauce ingredients are simmering, prepare stuffing.
Heat 2 qt water in large pot to boiling.
Add chicken, onion, garlic, and salt to taste.
Simmer just until chicken is cooked through, about 30 minutes.
Let cool, then remove chicken from liquid.
Remove and discard skin and bones; shred meat.
Prepare tortillas: When the sauce is nearly ready and the stuffing is ready, heat small amount of oil in skillet over medium heat.
Cook tortillas one at a time, turning once, just until flexible but not crisp, about 20 seconds each.
Drain on a thick layer of paper towels wrapped up inside a clean cloth kitchen towel (or tortilla warmer).
This will drain them and keep them soft and warm.
Heat oven to 3500°F.
Coat two 13x9 baking pans with vegetable oil.
Dip each tortilla in sauce.
Fill tortillas with shredded chicken and roll up.
Arrange a dozen filled tortillas in each pan.
Top with green sauce.
Spoon creme fraiche (or sour cream) over sauce.
Sprinkle chopped onion and cilantro over the top.
Sprinkle cheese over all.
Bake for 25 minutes.
Broil briefly if the top needs some browning.
Serve hot with refried beans.
To freeze: Prepare recipe through step 26. Place rolled enchiladas on a baking sheet and freeze until firm. Transfer to a freezer bag. Seal, label and freeze. Place sauce in a second freezer bag and place both bags in a larger bag. Seal and freeze.
To serve: Thaw in fridge overnight. Complete assembly and bake as directed in recipe.