I recently ate at a sushi restaurant where we just asked for "chef's choice," so we ended up with some dishes and rolls I was not familiar with. The first one was small pieces of various fish tossed together with a light sauce. Is there a name for this dish/preparation?
In general, slices of raw fish or other meats are called sashimi. These are often served un-sauced, but occasionally they are served brushed with any number of soy-based sauces.
From your description, you could also be describing a Japanese-inspired version of a dish very common in Hawaii called Poke. Poke is most often made with tuna, but could also be made with other fish or seafood. The other ingredients included in Poke are highly variable.