This hearty casserole is a favorite in Ireland, where it was whipped up during the coldest of winters. As its name suggests, it was originally made with lamb or mutton, but these days you may be more familiar with it being made with beef. For a healthier version, try using ground chicken.
Combine all of the ingredients in large Crock-Pot. Set on either low for 8 hours or high for 4 hours, taking care to not lift the lid during cooking time.
Meanwhile, place the cubed potatoes in a large pot of cold, salted water. Bring the pot to a boil and let it gently boil until the potatoes are fork-tender, about 12 minutes.
Drain the potatoes and return to the pot. Place the butter, seasoning, and truffle oil inside the pot with the potatoes and beat together with a hand mixer. Slowly add the milk as you are mixing, until you reach a desired consistency.
Preheat the oven to 350 degrees.
In an oven-safe baking dish, pour the stew in and spread it to evenly distribute. Spread a layer of the mashed potatoes over it. Brush lightly with milk. Place in the oven, uncovered, and bake for up to 30 minutes, or until the mashed potatoes are lightly browned. Let stand for 10 minutes before serving.