Cook the potatoes: Place peeled, quartered potatoes in a pot of cold salted water. Bring to a boil, cover, and cook until fork-tender.
Prepare the filling: Meanwhile, heat olive oil in a large pan over medium heat. Sauté onion for 2–3 minutes, add garlic and carrots, and cook 5–7 minutes until softened.
Add ground lamb and cook until no longer pink, about 5 minutes. Season with salt, pepper, rosemary, and thyme. Stir in tomato paste and flour until coated.
Pour in beef broth and peas. Simmer gently, stirring, until the sauce thickens. Remove from heat.
Mash the potatoes: Drain potatoes and transfer to a large bowl. Mash with butter, milk, cheddar, egg yolk (if using), salt, and pepper until smooth and creamy.
Assemble: Preheat oven to 400°F (200°C). Spread the meat mixture evenly in a 13x9-inch (33x22cm) baking dish. Top with mashed potatoes, smoothing the surface. If desired, use a fork to create a decorative pattern on top, which will brown beautifully in the oven.
Bake: Bake for 20–25 minutes, or until the potato topping is lightly golden and crisp. Let rest 5 minutes before serving.
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Notes
Meat options: Traditionally made with lamb, but ground beef or a lamb/beef mix works just as well.
Veggie swaps: Try adding corn, celery, mushrooms, or green beans for extra flavor.
Make-ahead: Assemble up to 24 hours in advance, cover tightly, and refrigerate. Bake just before serving.
Freezing: Shepherd’s pie freezes beautifully. Cool completely, wrap well, and freeze for up to 3 months. Bake from frozen at 350°F (175°C) for 45–55 minutes.
Potato topping variations: For extra creaminess, use half-and-half instead of milk. For a lighter option, swap in cauliflower mash.
Cheese lovers: Add extra cheddar or Parmesan on top before baking for a golden crust.
Texture tip: Use a fork to create ridges in the mashed potatoes before baking; they crisp up beautifully in the oven.
Leftovers: Store in an airtight container in the fridge for up to 4 days. Reheat in the oven for best results.