Trim any dark areas off the tuna.

Spread both sides with a thin film of the mustard.

Combine the peppercorns and poppy seeds on a flat plate and press them onto the tuna on both sides.

Wrap up the coated tuna smoothly in a sheet of foil.

Heat a heavy nonstick pan over medium heat.

Place the slab of tuna still wrapped in the foil in the pan and cook for 1½ minutes on one side. Turn it over and cook it for 1 minute on the other side.

Remove it from the pan.

Refrigerate the tuna until 1 hour before serving.

Place it in the freezer for 1 hour to make it easier to slice.

To serve remove the foil, cut the tuna into slices about ¼ inch (6mm) thick, and arrange them on 4 plates.

Drizzle with basil oil and serve garnished with lemon wedges and, if desired, some pecorino cheese.