Preheat oven to 375°F (190°C).
In a large skillet, heat vegetable oil over medium heat. Add diced onion and cook for 3-4 minutes until softened.
Add ground beef to the skillet with the onions. Cook, breaking up the meat with a spoon, until browned and cooked through, about 6-8 minutes.
Add minced garlic, salt, black pepper, and dried thyme to the beef mixture. Stir and cook for 1 minute until fragrant. Remove from heat.
In a large bowl, combine condensed cream of mushroom soup, sour cream, milk, and shredded cheddar cheese. Mix well.
Add the cooked ground beef mixture to the bowl with the soup mixture and stir until fully combined.
Lightly grease a 9×13 inch casserole dish. Spread the beef and sauce mixture evenly in the dish.
Arrange the frozen tater tots in a single layer over the beef mixture, covering the entire surface.
Cover the casserole dish tightly with aluminum foil.
For freezing: Place the covered casserole in the freezer. When ready to bake, thaw overnight in the refrigerator.
Bake the casserole uncovered at 375°F (190°C) for 35-40 minutes until the tater tots are golden brown and crispy and the casserole is bubbly.
Let the casserole cool for 5 minutes before serving.