Preheat oven to 375°F. Grease a 9x13-inch baking dish.
In a large skillet, cook ground beef and onion over medium heat until beef is browned and onion is softened, about 8 minutes. 1 1/2 pounds ground beef,1 medium yellow onion.
Add garlic and cook for 1 minute. Drain excess fat.3 cloves garlic.
Stir in ranch seasoning, Worcestershire sauce, cream of mushroom soup, frozen corn, milk, and thyme. Simmer for 3–4 minutes until well combined. 1 (1-ounce) packet ranch seasoning mix,1 tablespoon Worcestershire sauce,2 (10.75 ounce)cans cream of mushroom soup,1 cup frozen corn,1/2 cup milk,1/2 teaspoon dried thyme.
Spread the beef mixture evenly in the prepared baking dish and sprinkle all of the cheddar cheese over the top.2 cups shredded sharp cheddar cheese.
Arrange tater tots in a single layer over the top of the cheese.1 (32 ounce) bag frozen tater tots.
Cover with aluminum foil and bake for 25 minutes.
Remove foil and bake uncovered for 10–15 minutes, until the tater tots are golden brown and crisp.
Let cool for 5 minutes, then garnish with chopped chives if using. Serve warm.
Notes
Pro Tip: Don't skip covering the casserole with foil for the first part of baking! This helps the beef mixture stay moist while the tater tots start cooking. The total cook time should be around 35-40 minutes in a preheated oven. Removing the foil for the final bake ensures those cheesy tater tots get perfectly golden and crispy on top.