Searing meat to "seal in the juices" is an old wives' tale that has been debunked a gazillion times. You can find actual test results on it all over the internet from folks like Alton Brown, Kenji Alt @ serious eats, America's Test Kitchen, and many more.
Getting a nice brown crust does a lot for the flavor but does absolutely nothing about sealing anything.
Resting the meat after cooking, for as long as 30 minutes on a large roast, is the way to let the juices re-equalize within the meat. This helps keep them from spilling out on the cutting board. If you get more juice running than you want, return it to the roaster to contribute to a jus.
These same food science geeks have all also come to the conclusion that cooking a large roast low and slow does a better job of cooking the hunk of flesh evenly all the way through to your desired temperature, and retaining juices. Searing after the meat is cooked, via broiler, skillet or blowtorch will give you a flavorful brown crust without further cooking the interior.
From personal experience (and it's been confirmed by these same science-y folks) the best way to retain juice and cook evenly is to cooked the roast sous vide - under vacuum in a temperature controlled water bath. Brown it after for the desired Maillard reaction.