Preparation

Cut avocado in half (reserve remaining half by drizzling with lemon
juice). Combine peeled avocado half and next six ingredients 
(avocado through sour cream) in a blender. Purée until smooth, 
scraping down sides when necessary. Additional water may be
needed to thin sauce. Set aside.

Heat grill to 350-400 degrees. Place soaked plank on
grill, heat for 3 minutes, and turn. Continue to heat other side for an
additional three minutes. Brush both sides of fish with olive oil and 
place on hot plank. Close grill lid and cook for 12-15 minutes or until
fish flakes easily with fork. Top halibut with avocado sauce, reserved
avocado half thinly slice, and chopped fresh tomato. 
Yields: 4 servings