I use olive oil for both pans, but not too much; just a little will be enough.
Fry the small potato cubes with a little garlic, salt and pepper.
When they are nearly soft inside and crispy and crunchy outside,
Start warming well the pan for the tuna steak; it must be very hot, with just a tablespoon of olive oil.
Fry the steak thoroughly, all covered with the spice and the seasoning mixture, on one side for about 5 minutes (depending on its size and weight).
Then turn it and let it fry, always at a very hot temperature for another 3 minutes.
Add all of the glass of white wine and let it evaporate.
But take your tuna steak out from the pan before all the wine has evaporated, or else it will be overcooked.
It must remain rare and pink inside.
Wrap it in aluminium foil to keep it warm.
When the wine is nearly all evaporated, add a spoon of fish bouillon ,
If you don't have this, a vegetable bouillon will be good as well.
Cut the tuna steak in slices and serve it immediately, covered with the sauce you have made in the pan.