Preheat oven to 400°F and grease the baking dish lightly with oil or nonstick spray.

Warm the skillet over medium-high heat with 1 tbsp olive oil. Add the chopped onion and cook for 3-4 minutes until translucent.

Add the ground beef and cook, breaking it up with a spoon, until no pink remains - about 6-8 minutes. Drain excess fat if necessary.

Stir in minced garlic, Worcestershire sauce, thyme, salt, and pepper. Cook for 1 minute until fragrant.

Reduce heat to medium. Stir in the cream of mushroom soup and 1/2 cup milk until smooth.

Add the frozen mixed vegetables and 1 cup of shredded cheddar cheese. Cook for 2-3 minutes to heat through and melt cheese slightly. Taste and adjust seasoning.

Transfer the beef and vegetable mixture into the prepared 9x13 dish, spreading it into an even layer.

Sprinkle a thin layer of the remaining cheddar cheese over the filling.

Arrange frozen tater tots in a single, even layer across the top so they touch.

Place the casserole in the preheated oven and bake for 25-30 minutes, until the tots are golden brown and the filling is bubbling along the edges.

For an extra-crispy finish, switch the oven to broil for 1-2 minutes at the end, watching closely.

Let the casserole rest for 5 minutes after removing from the oven. Garnish with chopped fresh parsley if desired, then scoop and serve warm.