Brown beef on medium-high heat. Add salt and pepper to taste. Add onion and cook until softened.
Pour 4 oz of enchilada sauce on the bottom of a 9×13 glass casserole dish.
To assemble enchilada, lay out the flour tortilla and put about 1/4 cup of beef down the middle. Add a few tablespoons of cheese on top. Roll the tortilla and place in the baking dish seam side down. Continue until you have finished using all of the meat and tortillas.
Pour the remaining enchilada sauce on top of the tortillas. Then, sprinkle the remaining cheese on top. Cook for about 20 minutes, or until the sauce is bubbling and the cheese is a golden brown. Garnish with green onions and cilantro. Serve with sour cream if desired.