Preheat your oven to 375°F (190°C). While the oven warms up, gather and prepare all your ingredients so everything is ready to go.
In a large skillet over medium-high heat, add the ground beef. Season with kosher salt and black pepper. Break it up with a wooden spoon as it cooks. When the beef is about halfway browned, add the diced onion and minced garlic. Continue cooking until the beef is fully browned and the onions are soft, about 5-7 minutes. Drain excess fat if needed.
Sprinkle the all-purpose flour over the cooked beef mixture. Stir well to coat everything evenly and cook for 1-2 minutes to remove the raw flour taste. Slowly pour in the whole milk and beef broth while stirring constantly. Add the Worcestershire sauce. Continue to cook, stirring frequently, until the sauce thickens and becomes creamy, about 3-5 minutes.
Stir in the frozen mixed vegetables, allowing them to warm through and distribute evenly. Remove the skillet from heat, then fold in half of the shredded cheddar cheese until melted and incorporated.
Transfer the beef and vegetable mixture into a 9x13 inch (23x33 cm) baking dish, spreading it out evenly. Top with the frozen tater tots in a single layer, covering the entire surface. Sprinkle the remaining cheddar cheese evenly over the tater tots for an extra cheesy crust.
Place the casserole in the preheated oven and bake for 35-40 minutes, or until the tater tots are golden brown and crispy, and the filling is bubbling around the edges.
Remove the casserole from the oven and let it cool for 5 minutes before serving. This allows the sauce to set slightly and makes scooping easier. Serve warm and enjoy your homemade comfort food classic!