Pour ½ cup of Hatch Red Enchilada Sauce into the bottom of a 9-by-13-inch baking dish. Spread it evenly to coat the bottom.
In a large mixing bowl, combine cooked brisket with ½ cup enchilada sauce, 1 cup cheese, and the onion.
Wrap the tortillas in a damp paper towel. Microwave them for 30–45 seconds until warm and pliable so they do not crack.
Place a large spoonful of the brisket mixture down the center of each tortilla. Roll the tortilla up tightly. Place it seam-side down into the prepared baking dish. Repeat until the pan is snugly packed.
Pour the remaining 2 cups of Hatch Red Enchilada Sauce evenly over the top of the enchiladas. Ensure all edges are covered so they do not dry out.
Sprinkle with the remaining 2 cups of Mexican blend cheese generously over the sauce.
Bake uncovered for 20 to 25 minutes until the sauce is bubbling at the edges and the cheese is completely melted and lightly golden.
Let cool for 5 minutes before serving. Top as desired and serve.