Preheat and prep: Heat the oven to 400°F (200°C). Lightly grease a 9x13-inch baking dish or similar casserole.

Boil the potatoes: Add potatoes to a large pot, cover with cold salted water, and bring to a boil. Simmer 12–15 minutes, until very tender when pierced.

Make the mashed topping: Drain potatoes well. Return to the pot, add butter, warm milk, garlic powder (if using), and season with salt and pepper. Mash until smooth but not gluey. Adjust milk for creaminess. Set aside.

Start the filling: In a large skillet over medium heat, warm the olive oil. Add onion and cook 4–5 minutes until softened. Stir in garlic and cook 30 seconds.

Brown the meat: Add ground lamb (or beef/turkey). Season with salt and pepper. Cook, breaking it up, until browned and no longer pink, about 5–7 minutes. Drain excess fat if needed.

Build flavor: Stir in tomato paste, Worcestershire, thyme, rosemary, and smoked paprika. Cook 1–2 minutes to toast the tomato paste slightly.

Add veggies and thicken: Add peas and mixed vegetables. Whisk cornstarch into the broth, then pour into the skillet. Simmer 3–5 minutes, stirring, until the mixture thickens and looks saucy, not soupy. Taste and adjust seasoning.

Assemble: Spread the filling in an even layer in the baking dish. Spoon mashed potatoes over the top in dollops, then gently spread to cover, sealing to the edges to prevent bubbling over. Rough up the surface with a fork for browning. Sprinkle cheese if using.

Bake: Place on a rimmed sheet pan (for drips) and bake 20–25 minutes, until the top is lightly golden and the edges are bubbling. For extra color, broil 1–2 minutes at the end—watch closely.

Rest and serve: Let the pie rest 10–15 minutes before scooping. Garnish with parsley if you like. Serve warm.