There are so many ways to make and assemble enchiladas. Consider the green chile sauce in this recipe a starting off point and make them to your liking.
Heat a large skillet or pan on the stovetop with enough oil to coat the bottom of the pan. Add chopped onions and sauté until translucent. Then add minced onion and green chile. Mix and allow to cook a few minutes. Season with plenty of salt & pepper. Add hot water and let it come to a boil. Then lower heat and let it continue to simmer. Meanwhile, in a separate, small skillet, melt butter and add flour. Mix continually to form a paste, which will serve as thickening agent. Let cook for 2-3 minutes, then take paste and add it to pan with chile. Stir to let paste fully dissolve into sauce. Sauce will begin to thicken. Continue to let simmer on low. (Total amount of cook time should be about 15-20 minutes)
Meantime, fry corn tortillas and set aside. Place a layer of fried corn tortillas in a 9x13" baking dish. Pour enough chile sauce to completely cover them. Add shredded chicken. Top with shredded cheese, and more diced onion if desired. Then repeat the process, layering more tortillas, chile sauce, cheese and onion...until baking dish is full. Bake at 350º for 30 minutes. Remove from onion, let cool slightly before serving, and enjoy!