Place the peeled, chunked 2 ½ lb Yukon Gold potatoes peeled and cut into 1-inch chunks in a large pot and cover with cold 1 tsp kosher salted water by about 1 inch. Bring to a boil over high heat, then reduce to a steady simmer. Cook for 15–18 minutes, or until the 2 ½ lb Yukon Gold potatoes peeled and cut into 1-inch chunks are completely tender when pierced with a fork.

Drain the 2 ½ lb Yukon Gold potatoes peeled and cut into 1-inch chunks thoroughly in a colander and return them to the hot pot. Let them steam-dry for 1–2 minutes — this removes excess moisture and makes for fluffier mash.

Add the 4 tbsp unsalted butter to the hot 2 ½ lb Yukon Gold potatoes peeled and cut into 1-inch chunks and mash until the 4 tbsp unsalted butter is fully melted. Pour in the warmed ½ cup whole milk and ¼ cup sour cream (if using), then mash until smooth and creamy. Season with 1 tsp kosher salt and ½ tsp black pepper. Cover and set aside.

Preheat your oven to 400°F. While the 2 ½ lb Yukon Gold potatoes peeled and cut into 1-inch chunks cook, heat the 1 tbsp olive oil in a large skillet over medium-high heat. Add the 1 ½ lb ground lamb and cook, breaking it up with a wooden spoon, until browned and cooked through, about 7–8 minutes. Drain off any excess fat, leaving about 1 tbsp in the pan.

Reduce the heat to medium. Add the diced yellow onion, carrots, and 3 stalks celery diced small to the skillet with the 1 ½ lb ground lamb. Cook, stirring occasionally, until the vegetables have softened, about 6–7 minutes.

Add the minced garlic, 2 tbsp tomato paste, thyme, and 1 tsp fresh rosemary finely chopped. Stir everything together and cook for 2 minutes until the 2 tbsp tomato paste darkens slightly and the 4 cloves garlic is fragrant.

Sprinkle the 2 tbsp all-purpose flour over the meat and vegetable mixture and stir well to coat everything evenly. Cook for 1 minute to remove the raw 2 tbsp all-purpose flour taste.

Pour in the stock and 1 tbsp Worcestershire sauce. Stir well, scraping up any browned bits from the bottom of the pan. Bring to a simmer and cook for 5–7 minutes, stirring occasionally, until the gravy has thickened to a rich, glossy consistency that coats the back of a spoon. Stir in the 1 cup frozen peas. Taste and adjust seasoning with 1 tsp kosher salt and ½ tsp black pepper.

Transfer the 1 ½ lb ground lamb filling to a 9x13 inch baking dish and spread it into an even layer.

Spoon the mashed 2 ½ lb Yukon Gold potatoes peeled and cut into 1-inch chunks over the filling. Use the back of a wooden spoon or a spatula to spread them evenly all the way to the edges, sealing in the filling. For a decorative finish, use a fork to create ridges across the surface — these will brown beautifully in the oven. Sprinkle the shredded ½ cup shredded sharp cheddar cheese on top if using.

Bake in the preheated oven for 20–25 minutes, until the 2 ½ lb Yukon Gold potatoes peeled and cut into 1-inch chunks topping is golden brown on the ridges and the filling is bubbling around the edges. If the top isn't as golden as you'd like, broil for 2–3 minutes, watching carefully.

Remove from the oven and let the pie rest for 5–10 minutes before serving. This allows the filling to settle slightly and makes for cleaner, easier portioning.