Clean tuna of any sinew or rough pieces. Cut into 4 equal rectangular blocks. Working with each block, slice each rectangle into 5 equal pieces. Take a wooden skewer and slide through sliced pieces so that the fish comes back to a solid rectangle. This is done so that you do not have to slice the tuna once it is cooked, but rather can remove skewer and separate grilled pieces. Season the tuna with salt and black pepper. Grill tuna over a wood fired grill for approximately 1 minute on each of the 4 sides of the rectangle. This will produce a rare to medium-rare piece of tuna. Adjust cooking times to suit your taste. Grill ciabatta crostini just until toasted. Spread grilled bread with pesto.