This one was a huge hit last night.
Marinade for the tuna (from ALDI, it's on sale this week) was:
1/2 cup tamari (or soy sauce)
2 Tbls sesame oil
1 Tbls minced garlic
1 finely chopped green onion
2 Tbls brown sugar
Add everything to a gallon zip-lock and agitate until sugar is dissolved...