Preheat oven to 350°F/180°C and grease a 9x13 inch casserole dish with cooking spray.
In a skillet, cook ground beef until it’s no longer pink and cooked through. Drain fat. Remove onto a plate and set aside.
Add onion, and cook until softened (2-3 minutes), add garlic and sauté for 30 seconds – 1 minute.
Add flour, and cook it for 30 seconds then add the milk, beef broth, and Worcestershire sauce. Whisk and cook on medium heat for 3 minutes or until the sauce has thickened.
Stir in the beef and frozen vegetables and remove from heat.
Layer the beef in the casserole dish, followed by the cheese, and arrange the tater tots on top in a single layer.
Bake for 30-40 minutes. Transfer to a cooling rack, and cool for 15 minutes before serving. Garnish with freshly chopped parsley or chives.