Assuming this is stick, and that you can weld from the outside of the joint:
Take your time with the root pass, burn it in. You'll need to make sure you got the pieces lined up properly, too. Then burn the root pass. After that, do two cover passes over it, making sure you go slow enough that two passes will fill it in. (The black on the picture)
Then, you put another set of cover passes over that, making sure you catch the corners. That should take at least 3 passes. Watch the heat build up.
This may not be the "right" thing to do, but it's what I would do.