A couple of ways, Steve. Much like OG's idea, which works well with traditional taps.
With a monobloc, put your hand over the spout to stop any water coming out, then open up H and C. Depending on the type of mixer, the cold mains pressure should push water into the Hot side and clear the airlock.
If this doesn't work with your mixer, with the Cold shut off, open the Hot and blow hard up the spout.......... IF you can get your mouth over it!
Always worked a treat with old-type mixers, but can be difficult with some modern ones.