Cottage Pie is like Shepherd's Pie but with beef instead of lamb or mutton. I made this up to use leftover roast beef from Christmas. The roast was a prime rib, which made for a delicious pie, but other cuts will do. I wouldn't use chuck, but round or rump or sirloin roast should be fine, so long as it's not too tough a cut. Amounts are approximate; this isn't meant to be a very precise recipe. Times also are approximate.