Cooking different items in same oven - how to decide temperature?

What is the side dish?

au gratin potatoes is the side dish

I would start the potatoes at 325 then take out the roast to rest for 20 minutes when it is done, crank up the oven to 350 and finish the potatoes at that temp.

if its au gratin potatoes you should know when they are done
cuz they will be all golden-y brownish

Yup – she’s right.

I’m not the greatest of cooks – any idea time-wize? Should I taste it?

yes most defiantly taste it

and time wize im not sure

for time on the rib roast it depends on how thick it is and stuff like that

but my friend here says about an hour
and about 30 mins with the potatoes

Excellent. Thank you so much.

For a prime rib to cook in a 350 degree oven, it should take 23 -27 minutes per pound, depending on how well done you want it to be. Once you take it out of the oven, tent it with foil, and it will continue to cook for about 10 minutes. With potatoes au gratin in the same oven, you will be introducing an element of steam from the potatoes into the oven, so the meat will cook a little faster. The potatoes may take a little longer than if they were in the oven by themselves, because the presence of the prime rib will interrupt the circulation of the heat.

When I cook a whole tenderloin, I usually put the meat in at 450 degrees for 10 minutes, then turn the heat down to 325 for 25 minutes, then let it sit for 10 minutes before cutting. If I’m making scalloped potatoes with it, I usually put the potatoes in for 30 minutes, take them out, turn the heat up to 450, put the meat in for 10 minutes, turn the heat down to 325 and put the potatoes back in the oven. Once the meat comes out, I check the potatoes and usually leave them in for the additional 10 minutes while the meat finishes on the top of the stove. At this point, sometimes I cover the potatoes with foil if they are starting to get too brown.

Heat rises. The top of the oven will be hotter. Cook the side dish above the roast.