Preheat oven to 375°F (190°C). Heat oil in a skillet over medium; sauté onion until soft.
Add garlic and minced chicken; cook, breaking up meat, until no longer pink.
Stir in cumin, chili powder, black beans, corn, and 1 cup enchilada sauce; simmer 3–4 minutes. Season.
Spoon filling into tortillas, roll, and place seam-side down in a 9x13-inch baking dish.
Pour remaining sauce over enchiladas, sprinkle with cheese, and bake 15–20 minutes until bubbly.
Garnish and serve with sour cream or avocado if desired.