Make the Mashed Potatoes
Boil potatoes in salted water until fork-tender (about 15 minutes). Drain well.
Mash with butter, milk, and salt until smooth and fluffy. Set aside.
(Pro tip: slightly thicker mashed potatoes work best so they hold their shape on top.)
Make the Filling
Preheat oven to 400°F.
In a large skillet over medium heat, cook the ground lamb until browned. Drain excess grease if needed.
Add onion and cook until softened, about 3–4 minutes. Stir in garlic and cook another 30 seconds.
Mix in tomato paste, Worcestershire sauce, thyme, salt, and pepper.
In a small bowl, whisk together the bistro gravy mix and water (or broth). Pour into the skillet and simmer 3–5 minutes until thickened.
Stir in peas and carrots. The mixture should be thick and rich — not soupy.
Assemble
Spread the meat mixture evenly into a baking dish (or keep it in an oven-safe skillet).
Spoon mashed potatoes over the top and gently spread. Use a fork to create little ridges — those golden peaks are everything.
If using cheese, sprinkle over the top.
Bake
Bake for 20–25 minutes until bubbling around the edges and golden on top.
For extra browning, broil for the last 2–3 minutes (watch carefully).
Let rest 5–10 minutes before serving.