Heat the olive oil in a large skillet over medium heat. Add the onion and carrots and cook for 8 to 10 minutes, until soft but not browned.
Increase the heat to high, add the lamb, and cook until it is well-browned. Stir in the tomato paste and mustard, then add the stock. Reduce the heat to low, season with salt and pepper, and simmer for 15 to 20 minutes, or until the stock is mostly (but not completely) evaporated.
Transfer the meat mixture to a 1½-quart casserole dish. Cover with the mashed potatoes. Brush the top of the mashed potatoes with the melted butter. Bake for 50 minutes, or until evenly browned on top. Leftovers can be stored in the refrigerator for up to 4 days.
You can substitute ground beef for the lamb.
Assemble up to two days in advance, cover and refrigerate. Add 15 to 20 minutes to the baking time if going straight from the refrigerator.
To freeze: bake and cool completely, then cover in plastic wrap and aluminum foil and freeze for up to 2 months. Thaw overnight in the refrigerator, then reheat at 350 degrees for 1 hour, or until warmed through.