At least 1 hour before grilling, soak plank in enough water to cover. Place a weight on plank so it stays submerged during soaking.
In a small saucepan combine port and honey. Bring to boiling, stirring to dissolve honey. Reduce heat to medium; simmer, uncovered, about 15 minutes or until reduced to 1/4 cup. Cool.
In a small bowl combine garlic, thyme, salt, pepper, and coriander. Sprinkle mixture on both sides of steaks; rub in with your fingers.
For a charcoal or gas grill, place plank on the rack of an uncovered grill directly over medium heat until plank begins to crackle and smoke. Meanwhile, place steaks on the grill rack. Grill for 4 to 6 minutes or until grill marks appear, turning once halfway through grilling. Place steaks and onion slices on plank. Cover; grill for 9 to 11 minutes for medium rare (145 degrees F) or 11 to 13 minutes for medium (160 degrees F).
Transfer plank with steaks to a serving platter. Cut steaks in half. Separate onion slices into rings. Drizzle steaks with port mixture. Sprinkle with blue cheese and top with onion rings.