Peel potatoes and cut in half or quarters (depending on size). Peel carrots and cut into 2-inch chunks. (If you use an onion, cut it into 6-8 wedges. It's okay to cut the potatoes and carrots up the night before, cover them water and keep them in the fridge, but not the onion.) *Note: Use only a SMALL onion--it is just to mollify people who think it can't be pot roast without an onion, and a big one will make the taste too strong.