Season roast with salt and pepper. Cook in crock pot or oven (low heat for 4-5 hours) until tender. Shred beef. This step may be done the day before.
Saute onion in oil until tender. Stir 1/2 onion into shredded beef.
Prepare envelopes of enchilada sauce mix according to directions. Stir 1/2 cup sauce mix into shredded beef to moisten. Stir reserved onion into remaining sauce. Dip each tortilla in sauce and then fill with approximately 1/4 cup of beef mixture. Roll and place seam side down in greased pan. (Bottom of oven broiler pan works great.) When all are rolled cover with cheese. Bake at 350 degrees for 30-45 minutes.
