Ninben Tsuyu No Moto (Seasoning Soy Sauce). If you can't find it its just Dashi and Soy.
I don't have exact measurements of each ingredient. I just go by feel. Just take it easy on the sugar. Have enough liquid to cover the duck breasts.
Once cold slice and serve with rice and daikon radish.
Sushi Koto serves this as an appetizer. I think it goes well on top of a green salad or even in a sandwich, just serve in crusty French bread, arugula, tomato and a tiny bit of Japanese mayo.
I have yet to try it with wild duck as I didn't manage to get any waterfowl hunting done last season. No bad weather means no waterfowl.