Heat a large oven-safe skillet over medium-high heat. Brown the 1.5 lbs ground beef for about 5 minutes, drain most of the fat, then add the diced onion and carrots and cook for 4 to 5 minutes until softened. Stir in the 3 cloves minced garlic for 30 seconds.

Stir in the 2 tbsp tomato paste and cook for 2 minutes. Add the 1 tbsp flour and stir for 1 minute. Pour in the 2 tbsp Worcestershire sauce and 1 cup beef broth, add the 1 tsp dried thyme, and simmer on medium-low for 8 to 10 minutes until thickened. Stir in the 1 cup frozen peas, season with salt and pepper, and remove from heat. Preheat the oven to 400°F (200°C).

Boil the 2 lbs potato chunks in well-salted water for 15 to 18 minutes until fork-tender. Drain, return to the hot pot for 1 minute, then mash with the 4 tbsp butter, 1/2 cup warmed milk, 1/2 cup sour cream, and 1/2 tsp garlic powder. Season with salt.

Spread the filling in a 9x13-inch baking dish, top with the mashed potatoes, spread to the edges, and drag a fork across the surface to create ridges. Bake at 400°F (200°C) for 25 to 30 minutes until the edges bubble and the top is golden.

Scoop into wide bowls and top each serving with fresh parsley or chives and a crack of black pepper.