Scrub and peel the potatoes, chop into 1-inch pieces, and place in a large pot of cold water. Bring to a boil over high heat and cook for 12 to 15 minutes until very tender when pierced with a fork but not falling apart. Drain well.

Return potatoes to the pot. Add butter and mash until smooth. Stir in milk, sour cream, salt and pepper. Taste and adjust seasoning. If the potatoes feel stiff, add another splash of warm milk and stir until creamy. Cover and set aside.

Preheat oven to 400 degrees F.

Heat a large skillet over medium-high heat. Add ground beef and chopped onion. Cook breaking up the meat for 6 to 8 minutes until browned with no pink remaining. Drain off excess grease.

Reduce heat to medium. Add garlic, frozen vegetables, flour and tomato paste. Cook stirring constantly for 1 to 2 minutes until everything is well coated.

Stir in beef broth, Worcestershire sauce, paprika, thyme, salt and pepper. Bring to a low simmer and cook for 10 minutes until most liquid is absorbed and filling is thick but still moist. Add a splash more broth if it looks dry. Remove from heat.

Transfer filling to a 9x13-inch baking dish and spread evenly. Spoon mashed potatoes over the top and spread into an even layer. Use a fork to create ridges across the surface.

Bake uncovered at 400 degrees F for 25 to 30 minutes until edges are golden brown and filling is hot and bubbling. Broil for 1 to 3 minutes for a more golden top if desired, watching closely. Let stand 5 minutes before serving.