Shepherd's Pie

Ingredients

• 2 pounds yukon gold potatoes – peeled and cut into chunks
• 2 teaspoons kosher salt
• 4 tablespoons unsalted butter
• 0.75 cup whole milk
• 1 large egg yolk – lightly beaten
• 1 tablespoon olive oil
• 1 medium yellow onion – finely chopped
• 2 medium carrot – finely diced
• 1 stalk celery – finely diced
• 2 cloves garlic – minced
• 1.5 pounds ground lamb
• 2 tablespoons tomato paste
• 2 tablespoons all-purpose flour
• 0.5 cup dry red wine
• 1 cup beef stock
• 1 tablespoon Worcestershire sauce
• 2 teaspoons fresh thyme leaves – chopped
• 1 teaspoon fresh rosemary – chopped
• 1 leaf bay leaf
• 1 cup frozen peas – thawed
• 0.5 teaspoon ground black pepper

Instructions

1. Heat the oven to 400°F. Place the yukon gold potatoes in a large pot and cover by 1 inch with cold water. Add 1 teaspoon of the kosher salt to the pot now, reserving the remaining kosher salt for seasoning the filling later; you will add all of the ground black pepper to the filling later. Bring to a boil, then simmer until the potatoes are very tender, 15–20 minutes.
2. While the potatoes cook, warm the whole milk with the unsalted butter in a small saucepan over low heat until the butter melts; keep warm off heat.
3. Make the filling: Heat the olive oil in a large skillet over medium-high heat. Add the yellow onion, carrot, and celery and cook, stirring, until softened and lightly browned, 6–8 minutes. Add the garlic and cook until fragrant, about 30 seconds.
4. Add the ground lamb and cook, breaking it up, until browned with no pink remaining, 5–7 minutes; spoon off excess fat if needed.
5. Stir in the tomato paste and cook until it darkens and smells sweet, 1–2 minutes. Sprinkle over the all-purpose flour and cook, stirring, 1 minute.
6. Pour in the dry red wine, scraping up browned bits, and let it reduce by about half, 2–3 minutes. Add the beef stock, Worcestershire sauce, fresh thyme leaves, fresh rosemary, and bay leaf; bring to a gentle simmer. Reduce heat to medium and cook until thickened to a saucy consistency, 8–10 minutes. Remove and discard the leaf, then stir in the frozen peas and cook 1–2 minutes. Season to taste, then take the pan off the heat.
7. Drain the potatoes well and return them to the pot over low heat for 30–60 seconds to evaporate excess moisture. Mash until smooth, then pour in the warm milk–butter mixture and beat until creamy. Stir in the egg yolk until the mash is glossy; taste and adjust seasoning.
8. Assemble in a 2 to 2.5-quart baking dish: Spread the lamb filling in an even layer. Dollop the mashed potatoes over the top, spread to the edges to seal, and rough up the surface with a fork to encourage browning.
9. Bake until the filling is bubbling at the edges and the top is lightly golden, 25–30 minutes. For deeper color, broil 2–3 minutes, watching closely. Rest 10 minutes before serving.