Combine first 6 ingredients in a large zip-top plastic bag. Add fish to bag; seal and marinate in refrigerator 1 hour, turning once.
Combine sour cream and next 4 ingredients in a small bowl; set aside.
Remove fish from marinade, reserving marinade. Place fish on grill rack coated with cooking spray; grill 6 minutes on each side or until fish is medium-rare or desired degree of doneness, basting frequently with reserved marinade. Serve with sour cream mixture.
Sauce was excellent, and would be good with beef also. I used yogurt instead of sour cream because that is what I had. Adjusted the other ingredients to my own tastes, i.e., more horseradish. I did not bother with the marinade -- just salted the tuna and let it sit for about 1/2 hour to get rid of any fishiness, then rinsed, patted dry, oiled lightly, and grilled. Delish.