Fire up the oven to a cozy 400°F. While it's warming up, give some love to a 9x13" casserole dish – a quick grease should do.
On the stove, medium heat is your friend. Brown up the ground beef and onion until there's no pink left. If there's any extra fat, drain it away.
Grab a bowl and do a happy dance by mixing soup, sour cream, milk, garlic powder, dry mustard, ½ cup of cheese, salt, and pepper. It's the magic sauce.
Back to the casserole dish – unite the beef, soup mix, and mixed veggies. Now, here comes the star – layer those tater tots on top like they're the VIPs and sprinkle the remaining cheese.
Wrap it up with foil and let it mingle in the oven for 30 minutes. Then, unveil the deliciousness, and let it bask in the heat for an extra 20 minutes until it's golden and bubbly.