I'm making a porterhouse roast. Is it okay to brown it on the stove, then let it sit for an hour before putting it in the oven to roast? Or does it need to go in the over right away?
If you were going to braise it, I'd say yes, ok (restaurants do this). But if you're roasting, I'd put it in the oven right away.
Don't sear your meat before briasing because restaurants do it. Do it because it's the right way to do it. Escoffier did this, all the best chefs in the world do it. It is a classic french culinary technique. The two main reason for searing before you briase is for flavor and juice retention. Yes, It's fine to sear an hour in advance or a day in advance.
But, if you mean roast it in dry heat to serve, let's say, medium rare then I personally wouldn't. If were talking about a 3-5 inch thick porterhouse then season and sear on both sides and pop it in a 425F oven and pull at around 150F at center and let stand 10 min or so before portioning.
Instruction for braising: Season, sear, partially cover with a thickened liquid, cook until done.
Intructions for dry heat roasting: Season, sear, cook to desired temp.
Just to clarify (!), not that you would ever braise a porterhouse -- I just meant in general, if you were braising a cut of meat, I think it would be ok to brown it and let it sit before proceeding. But not for a cut that you're roasting.