Heat oil in a large skillet over medium heat. Sauté onion until just tender and then add the ground beef. Cook, stirring until the beef is no longer pink. Drain and then add the chili powder, ground cumin, salt, and minced garlic. Cook for a minute or two longer, and then stir in tomato sauce; continue cooking until hot.

Make foil "handles." Fold a 24 to 30-inch long piece of foil in half lengthwise. Place in bottom of a slow cooker with both ends hanging over the top edge of cooker. The foil strip will make it much easier to remove the stack when ready to serve.

Lightly butter one side of the corn tortillas. Lay one tortilla, buttered side up, on the foil. Spread the tortilla with the meat mixture and a generous layer of shredded cheese. Cover with another tortilla (buttered side up), more meat mixture and cheese. Repeat layers, ending with cheese. Fold excess foil over the top of the stack, cover the slow cooker, and cook on HIGH for 1 hour.

When ready to serve, lift out, using foil strips and transfer to the serving dish. Let the taco stack cool slightly, and then cut into wedges.

Serve with sour cream, chopped green onion, and salsa or diced fresh tomatoes, as desired.