In a non-stick skillet, melt the butter over medium heat. Add the diced onion and cook until translucent, about 3–5 minutes. Remove from heat and let cool slightly.

In a large bowl, combine the shredded roast beef, sautéed onion, and 2 cups of the grated Mexican cheese blend. Mix until well incorporated.

Pour the red chile enchilada sauce into a shallow bowl. Dip each corn tortilla into the sauce so it’s coated but still pliable.

Place a portion of beef-cheese mixture in each tortilla and roll it up. Arrange the rolled enchiladas seam-side down in a 9 × 13-inch casserole dish (you should fit 8–10).

Pour any remaining red chile sauce evenly over the top of the arranged enchiladas. Sprinkle with the remaining 1 cup of cheese.

Preheat your oven to 350°F (175°C). Bake the enchiladas for 20–25 minutes or until the cheese is melted, bubbly, and slightly golden.

Remove from oven. Garnish generously with shredded lettuce, chopped tomatoes, cilantro, sliced black olives, and dollops of sour cream. Serve immediately.