Making a boneless prime rib roast. Help!?

No recipe I have found is anything like how I want to cook my prime rib roast for Tuesday and I want to make sure it will work. Here is what I want to do:

I want to slice up an onion or two (very thick slices) and line the bottom of my pan with them. Then, place the prime rib roast on top of the onions (remember, my roast is boneless) and just cook it that way so that the roast and the drippings have the onion flavor. But every recipe I've found for a boneless roast says to place it on top of a metal rack to cook, so I'm worried that not using a rack will somehow screw up my roast.

So, will using onion slices instead of a metal rack work?