Fill a large pot with water, add the potatoes, place over high heat and bring to a boil. Add some salt and cook until tender, about 12 minutes.
While potatoes are cooking, heat a large skillet over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons. Once hot, add the lamb and, using the back of a wooden spoon, break it up as it browns, about 5-6 minutes. Season with salt, pepper and allspice. Add carrot, parsnip and onion, and cook 5 minutes, stirring frequently.
Dust with flour, cook 1 minute, then add stock, beer and Worcestershire. Cook until thickened, then transfer to oven-safe pan.
Drain potatoes and return to the pot. Let sit a few minutes to cool slightly. Add sour cream or cream cheese, egg yolk and cream. Smash until smooth.