Adding 'swirls' to savoury bread dough
Any additional ingredients should be incorporated after the first proofing. 

So after your bread has been set aside to rise in a warm place and doubled in size, knock it back to release the carbon dioxide, then add in your mustard any way you want.

It might be worth rolling out the dough, spreading the mustard along the inside, and then rolling up the dough and cutting it into small pieces to create the swirls.

The bread should then be shaped as you want it and left to rise for another 30-45 minutes or so. After all this it should be good to go.