Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).
Place cubed potatoes in a pot of salted water and bring to a boil. Cook for 15 minutes or until tender, then drain.
Mash the potatoes with milk, butter, salt, and pepper until smooth.
In a large skillet, brown the ground meat over medium-high heat. Drain excess grease.
Add the diced onion to the skillet and sauté for 5 minutes until translucent.
Stir in the tomato paste, frozen vegetables, beef broth, Worcestershire sauce, and thyme. Simmer for 8 to 10 minutes until liquid is slightly reduced.
Transfer the meat and vegetable mixture into a 2-quart baking dish.
Spread the mashed potatoes evenly over the meat mixture, using a fork to create peaks on the surface.
Bake for 20 minutes or until the potato topping is golden brown and the edges are bubbling.