Season the tuna with salt and pepper and sear in a hot pan.
Tuna is best if cooked rare. It is such a lean fish that if it cooked more it is tough and the sweetness of the meat is lost.
The tuna can be used hot or chilled depending on your preference.
I like to slightly toast the bread and assemble the coleslaw on the bread, then chutney, then tuna, chutney, coleslaw and bread. Some like the coleslaw on the side. If you are a pumpernickel fan I would use it. If not, a nice Dutch crunch roll is good.