Add a little olive oil in a frypan when hot add the tuna steaks and cook over a medium low heat for 4 minutes. Turn the steak and cook for a further 4 minutes. The tuna should be slightly pink inside still when finished.
Remove the tuna steaks form the frypan; keep warm.
Add butter to the frypan, once melted add the mushrooms until cooked then move to one side.
Tip the frypan so the butter is on one side then add the flour and mix to a paste. Add some whole peppercorns and milk gradually making the paste thinner into a sauce.
Mix in the mushrooms then simmer for 2 minutes. This should give a nice peppery mushroom flavour.