Scrub the potatoes clean and add to a deep pot. Completely cover the potatoes with water, about ½ inch above the potatoes. Bring the potatoes to a boil and add ½ teaspoon of kosher salt. Boil the potatoes for about 20 minutes or until they are very easily pierced with a fork and the skin pulls away from the potato where cut.

Drain the potatoes, and while still hot, use a paper towel to wipe the skin away from the potatoes. Add them back to the warm pot.

Gently smash the hot potatoes with the beaters of a handheld mixer or a potato masher. Add the butter to the potatoes and beat or mash to break the potatoes down then add the half and half. Beat or mash the potatoes until smooth, adding more half and half if needed. Season with salt and pepper, adding more to taste. Use a wooden spoon to stir in the cheese and chives. Remove from heat, top with a lid, and set aside while you make the shepherd’s pie filling.

Preheat the oven to 400ºF.

In a 10-inch cast iron skillet (or another large oven-safe skillet or Dutch oven, that’s shallow and wide), add the oil and heat over medium. Once hot, add the carrots, celery, and leek, and sauté, stirring occasionally for about 10 minutes, or until the veggies are tender. Stir in the garlic and sauté for 2 minutes. Transfer the veggie mixture to a large bowl or plate and set aside.

To the same skillet, add the ground beef and season with salt and pepper. Break up the meat and cook until cooked through and no longer pink, about 5 minutes or so. Add the tomato paste and herbs and stir into the beef, cooking for 2-3 minutes. Add the vegetables back to the skillet. Stir the flour into the veggies and beef, coating them well, and cook for 1 minute. Add the beef stock, soy sauce, Worcestershire sauce, and peas, and bring the mixture to a boil. Once boiling, remove from heat. Taste the filling and add more salt or pepper if needed.

Even out the meat mixture in the skillet, then dollop the mashed potatoes on top. Use an offset spatula or wooden spoon to spread the potatoes into an even layer. Sprinkle with chives. Place the skillet in the oven, uncovered, to bake for 15 minutes. Raise the heat to broil and cook for 5 more minutes or until the mashed potatoes are lightly browned and the filling is bubbly. Remove from the oven and let sit for 10-15 minutes before topping with more chives and serving.