Place the potato chunks in a large pot, cover with cold water by about 1 inch, add a generous pinch of salt, then bring to a boil. Cook for 15 to 18 minutes, or until fork-tender. Drain and return to the pot.

Mash the potatoes with warmed milk, butter, sour cream, and salt until smooth and creamy. Set aside.

Preheat the oven to 400°F (200°C). Heat olive oil in a large oven-safe skillet over medium-high heat. Add ground beef and cook for 6 to 7 minutes, breaking into crumbles. Drain excess fat.

Add onion and carrots, cooking for 5 to 6 minutes until softened. Stir in garlic and cook for 1 minute.

Sprinkle flour and tomato paste over beef mixture, cooking and stirring for 1 minute. Slowly pour in beef broth and Worcestershire sauce while stirring. Let simmer for 4-5 minutes until thickened.

Stir in frozen peas, thyme, salt, and pepper, then remove from heat.

Spoon mashed potatoes evenly over the beef filling, spreading to edges. Create decorative ridges on top with a fork and sprinkle optional cheddar cheese.

Bake for 20 to 25 minutes, until filling is bubbling and potatoes are golden brown. Let rest for 10 minutes before serving.