Preheat your oven to 375°F. Place the cubed potatoes in a large pot, cover with cold water, and add a generous pinch of salt. Bring to a boil and cook for 15-20 minutes until fork-tender.
While potatoes are cooking, brown the ground beef in a large skillet over medium-high heat. Drain excess fat, then add the onion and cook until softened, about 5 minutes. Add the garlic and cook for another minute.
Stir in the tomato paste and flour, cooking for 1 minute. Add the beef broth, Worcestershire sauce, thyme, and frozen vegetables. Season with salt and pepper. Simmer for 5-7 minutes until the sauce thickens.
Drain the potatoes and return them to the pot. Add butter and milk, then mash until smooth and creamy. Season with salt and pepper to taste.
Pour the meat mixture into a greased 9x13-inch baking dish and spread evenly.
Spread the mashed potatoes over the meat layer, using a spatula to create an even layer. You can use a fork to create decorative ridges on top if you want it extra pretty.
Sprinkle the shredded cheddar cheese over the top.
Bake for 25-30 minutes until the cheese is melted and the edges are bubbly. For extra golden-brown topping, broil for 2-3 minutes at the end. Let rest for 5-10 minutes before serving.