It the fit up is pretty good with a consistent gap, a downhill root might be a good choice. After that, go uphill with a weave that is just wide enough to allow for a fairly flat weld.
Here is a video that might help that shows a downhill root pass open butt and also how to weld uphill over that root pass.
In this video flat plates are used but are gapped to simulate a beveled joint.
If more passes were needed, they could be stacked side by side