Add the onion, garlic, and beef broth into the bottom of a 6-quart crock pot. Trim away any excess pieces of fat from the roast, cut it into 6-8 big pieces, then nestle them on top of the onion mixture. Sprinkle in the chili powder, cumin, garlic salt, and pepper then place the lid on top and slow cook on low for 6-7 hours, or until the beef shreds easily with 2 forks. Transfer the beef to a cutting board then shred and place inside a large mixing bowl with enough cooking liquid to make it juicy, but not so much that it's dripping. Stir in the green chiles. Discard the remaining cooking liquid.
Preheat the oven to 350 degrees then spray a 9x13" baking pan, or two smaller pans, with nonstick spray. Spread 1/2 cup enchilada sauce in the bottom of the pan. Divide the shredded cheese in half, reserving half for topping the enchiladas.
Wrap half the tortillas in a damp paper towel then microwave for 30 seconds. Spread about 1/4 cup (eyeball it) shredded beef across the center of each tortilla then top with 1 Tablespoon shredded cheese and roll. Place seam side down in the prepared pan then repeat with remaining ingredients. Pour enough enchilada sauce over the top of the tortillas to cover, then top with the remaining cheese.
Spray a large piece of foil with nonstick spray then place sprayed side down on the pan and crimp shut. Bake for 20 minutes then remove the foil and bake for 5-10 more minutes or until the cheese is melted and the sauce is bubbly. Let the enchilada sit for 10 minutes before serving with toppings.
Notes
To freeze: Wrap the unbaked enchiladas with plastic wrap then foil, and then freeze. Thaw for 24 hours in the fridge then remove the plastic wrap, reapply foil that's been sprayed with nonstick spray, and then bake as directed above.