Toast cumin seeds in heavy small skillet over medium-low heat until aromatic, about 1 minute. Mince cumin seeds. Add cumin, chili mixture, tomatoes, quartered onion, garlic, sugar and salt to stock. Simmer until all ingredients are very soft, about 45 minutes.
Working in batches, puree stock mixture in blender. Return to pot. Boil until reduced to about 5 cups, stirring occasionally, about 20 minutes. Season with salt and pepper. (Can be made 1 day ahead. Chill chicken and sauce separately.) Remove skin from chicken and discard. Cut meat from bones and shred. Transfer to bowl and combine with 1/2 cup sauce. Set filling aside.
Oil two 13x9-inch glass baking dishes. Pour oil into deep skillet to depth of 1/2 inch and heat to 375F. Fry tortillas 1 at a time until softened, about 5 seconds per side. Using metal spatula, transfer to paper towels. Spread 1 tablespoon sauce over each tortilla. Sprinkle each with 2 tablespoons cheese and 1 tablespoon chopped onion. Place 1/3 cup chicken down center of each tortilla; roll up. Place seam side down in baking dishes.
Preheat oven to 350F. Pour remaining sauce over enchilada. Sprinkle with remaining cheese. Bake until heated through, about 20 minutes. Top with creme fraiche, avocado and cilantro.