Rinse stalks under cold running water and scrub the ridges clean. Trim the base and any discolored ends. Slice on a bias or into uniform pieces for even cooking — thin slices for fast sautés, half-inch pieces for longer covered cooking. Use inner stalks if working with conventional celery; they are less fibrous. Have all aromatics prepped before the pan goes on.

Set a large Skillet over medium heat for softer results or medium-high for a quicker, crunchier sauté. Add butter or oil and heat until shimmering or foamy before adding celery. Do not crowd the pan or the celery will steam instead of sauté. Add celery to the hot fat. If cooking with onion, add it first and cook about 5 minutes until translucent, then add celery. Add garlic or scallions after the celery has cooked 1 minute, then cook 30 seconds more. Stir occasionally to allow some color to develop. For tender-crisp celery, cook 2 to 3 minutes over medium-high heat. For softer celery, cook 5 to 10 minutes over medium heat. To go fully tender, add a splash of water, cover, and cook an additional 5 minutes. Total time is roughly 6 to 10 minutes depending on thickness and desired texture.

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 warmed and ¼ cup sour cream (if using), then mash until smooth and creamy. Season with 1 tsp kosher salt plus more and ½ tsp black pepper freshly ground. 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 1 yellow onion finely diced, 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 4 cloves garlic minced, 2 tbsp tomato paste, thyme, and 1 tsp fresh rosemary finely chopped, or ½ tsp dried. Stir everything together and cook for 2 minutes until the 2 tbsp tomato paste darkens slightly and the 4 cloves garlic minced 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 plus more and ½ tsp black pepper freshly ground.

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.