Combine all ingredients in a saucepan and heat to just below the boil. Remove from the heat. Cool. This marinade may be stored in a refrigerator up to 2 weeks. Use to marinate chicken, shrimp, or pork for several hours before grilling. Yield: 1 1/2 cups
Cut the tuna into 24 equal-size cubes. Quarter the mushrooms. Cut the peppers and onion into 1/2-inch squares. Divide the ingredients equally among the 8 skewers, alternating squares of vegetable and tuna. Pour 1 cup of the Sherry-Soy Marinade over the kebabs and marinate, refrigerated, 2 to 3 hours.
Dissolve the wasabi in the remaining 1/2 cup marinade to make a spicy dipping sauce to serve at the table with the grilled tuna.
On a hot outdoor grill or under a broiler, sear the kebabs 2 to 3 minutes on each side. The tuna should be medium rare--still red in the center--when served with the dipping sauce.