Preheat your oven to 350 degrees F.
In a large skillet, heat olive oil over medium heat. Once hot, add the finely chopped onion and carrots. Cook for about 8 to 10 minutes until they are soft but not browned.
After the vegetables are soft, increase the heat to high. Add the ground lamb and cook until it is well-browned, stirring to break it apart.
Once the meat is browned, stir in the tomato paste and Dijon mustard. Mix well to combine everything.
Next, pour in the warmed stock. Reduce the heat to low, season generously with salt and pepper, and let it simmer for 15 to 20 minutes.
Transfer the meat mixture to a 1½-quart casserole dish, spreading it out evenly. Top with the mashed potatoes.
Brush the top of the mashed potatoes with the melted butter.
Bake in your preheated oven for about 50 minutes. Look for a beautifully browned top.
Finally, let it cool for a few minutes before serving.
