(*Helpful hint - if you are only in need of a 3-4 lb. roast and can't find one in the refrigerated section, take the largest one to the butcher's window and ask them to cut it in half for you. They will be more than happy to do so, and it will really help to save money on that shopping trip!)
I buy the pre-sliced onions sold in the refrigerated produce aisle. The cost is only slightly more, but is worth its weight in the tears, hard work and time it takes to cut up an onion!