1. Preheat the oven to 375 degrees Fahrenheit.
2. Heat 1 tablespoon of the butter in a pan. Add the beef and season with salt and pepper. Cook, stirring occasionally, until browned, about 10 minutes. Set aside.
3. Add the remaining butter to the skillet. Add the onion and season with salt and pepper. Cook, stirring occasionally, until slightly browned, about 10 minutes.
4. Add the garlic to the skillet and cook until fragrant, about 1 minute.
5. Return the beef to the pan. Sprinkle the flour over the top. Cook for about 1 minute, then stir in the beef broth, Worcestershire sauce and thyme. Bring to a boil, then add the frozen veggies.
6. Brush a baking pan with melted butter. Arrange about two-thirds of the potato slices in it, overlapping them slightly and bringing them up the sides of the pan. Brush with melted butter and season with salt and pepper.
7. Add the beef mixture to the baking pan. Top with the remaining potato slices, overlapping them slightly. Brush with melted butter and season with salt and pepper.
8. Bake for 40 to 45 minutes, until the potatoes are tender. Place the casserole under the broiler to get the topping nice and crisp.
