Place the potatoes in a large pot of cold water over medium heat. Bring to a boil and cook until tender, about 15 minutes. Drain reserving 1/2 cup of the cooking water.

Mash the potatoes with a fork or potato masher, adding the cooking water along with the butter, salt and pepper.

Beat with a wooden spoon until fluffy.

Preheat the oven to 400°F.

While the potatoes are cooking, heat the oil in a medium skillet over medium-low heat.

Add the onions carrots and celery, cook stirring occasionally, until softened, about 15 minutes.

Increase the heat to medium and add the lamb.

Cook for about 5 minutes, stirring to brown evenly. If using raw lamb, cook for 10 minutes more, stirring and breaking up the meat with a wooden spoon. Spoon off any fat. Add the flour and cook, stirring, for 2 to 3 minutes.

Add the stock, thyme, rosemary, nutmeg, salt and pepper.

Reduce the heat to low and simmer, stirring occasionally, until thickened, about 5 minutes. Let cool slightly, then remove to a 9-inch pie plate or baking dish. Spread the mashed potatoes over the top, making irregular peaks with the tines of a fork. Scatter over the top the meat mixute.

Bake until potatoes are browned and the dish is heated all the way through, 30 to 35 minutes. Let cool slightly, then serve directly from the baking dish.