01 - Preheat oven to 375°F. Grease a 9×13-inch baking dish with nonstick spray or butter.
02 - Heat a large skillet over medium-high. Add ground beef and cook, breaking into crumbles, until browned. Add diced onion and cook 5 minutes until softened. Stir in minced garlic, thyme, paprika, salt, and black pepper; cook 1 minute more. Drain excess fat.
03 - Stir in frozen mixed vegetables and cook for 2–3 minutes until heated through. Transfer beef-vegetable mixture to the prepared baking dish, spreading evenly.
04 - In a mixing bowl, whisk together cream of mushroom soup, cream of celery soup, sour cream, 1 cup shredded sharp cheddar, garlic powder, and pepper until smooth. Spread sauce evenly over beef and vegetable layer.
05 - Arrange frozen tater tots in a single layer over the sauce, fully covering the surface.
06 - Place uncovered dish in preheated oven. Bake 25–30 minutes until tater tots are golden brown and casserole bubbles at the edges.
07 - Remove from oven and sprinkle remaining 1 cup shredded cheddar cheese over the hot tater tots. Return to oven for 3–5 minutes until cheese melts. Let set for 5 minutes, then garnish with chopped chives or green onions before serving.