Preheat your oven to 375°F (190°C) so it will be ready once the casserole is assembled.

In a large skillet over medium-high heat, cook the lean ground beef and finely chopped yellow onion together until the meat is fully browned and no pink remains. Stir occasionally to break up the beef. Drain off any excess fat before proceeding.

In a medium bowl, combine the thawed or fresh mixed vegetables, condensed cream of mushroom soup, milk, dried parsley, dried oregano, a pinch of salt, and black pepper. Add the beef and onion mixture from Step 2 to the bowl, then stir in 1/2 cup shredded cheddar cheese. Mix everything thoroughly until evenly combined. I like to use Kerrygold cheddar here—it really bumps up the flavor!

Spread the beef and vegetable mixture from Step 3 evenly into a 3-quart or 9x13-inch baking dish. Sprinkle the remaining shredded cheddar cheese over the top to create a cheesy layer.

Arrange the frozen tater tots in a single, even layer on top of the cheese-covered mixture. Place the dish in the preheated oven and bake for 40 to 50 minutes, or until the sauce is bubbly and the tater tots are crispy and golden brown. For extra crunch, you can broil the casserole briefly at the end—just keep an eye on it.