Preheat oven to 400F. Position rack in top third of oven.
Boil potatoes in salted water 15-20min until tender.
Add the oil to a large skillet and place it over medium heat. Add the onions and cook until soft about 5 minutes, stirring occasionally. Add garlic, cook till fragrant.
Add the ground beef (or ground lamb) to the skillet and break it apart with a wooden spoon. Cook till no longer pink. Stir flour into meat, add stock slowly. Stir in salt & pepper, allspice, vinegar, Worcestershire and cook 1-2min.
Pour into 1.5qt casserole. Melt 1Tbsp butter and reserve to brush on taters. Mash taters with salt pepper, butter and milk (the mashed taters will be dense). Spread over meat and brush with butter.
Bake till top is golden brown, 25-30min
