Cook beef and onion in large skillet over medium high heat until meat is browned, stirring until crumbled; drain and return to skillet.
Stir soup and enchilada sauce into meat mixture. Season to taste with salt and pepper. Spread 1/4 cup meat mixture onto each tortilla; sprinkle tortillas evenly with 1 cup cheese. Roll up tortillas, place, seam side down, into 9x13 pan. Pour remaining meat mixture over tortillas.
Cover and bake at 350 for 20 minutes. Uncover and sprinkle with remaining cheese, bake 10 more minutes. Serve with sour cream.