Tuna must be grilled quickly, since it will dry out badly if overcooked—so watch it carefully. Of course you can also grill tuna under an oven broiler.
Whisk the marinade ingredients together in a large bowl until smooth. Add the tuna, fennel, and red onion, and turn in the marinade. Cover loosely, and refrigerate for several hours or overnight.
Prepare hot coals for grilling.
On four large metal skewers, thread the tuna chunks alternately with pieces of fennel and onion.