Preheat the oven to 375°F (190°C).
Grease a 9x13-inch baking dish with cooking spray or butter.
Melt butter in a large skillet over medium heat.
Add chopped onion and cook until soft.
Stir in minced garlic and cook for 1 minute.
Add the mixed vegetables and cook for 2–3 minutes.
Stir in cream of chicken soup, sour cream, and milk.
Add shredded chicken, salt, pepper, and 1 cup of cheese. Mix well.
Pour the chicken mixture into the baking dish and spread evenly.
Arrange tater tots in a single layer on top.
Sprinkle the remaining cheese over the tater tots.
Bake uncovered for 35–40 minutes, or until the top is golden and crispy.
Let the casserole rest for 5 minutes before serving.
Garnish with parsley or green onions if desired.