Ironchef, i've used purple peruvian potatoes before, but they were never THAT purple, I would love to know where you got them, I'd love to get some for Friday for the fish special. The tuna looks great.
Chef Jimmy, look for purple yams. It's an off-shoot of the Okinawan and Peruvian purple sweet potatoes. It's common in Filipino cuisine where it is called "Ube". Check with your purveyors or you might want to try browsing or calling up any Asian markets in your area. If you know of any Filipino stores that sell produce that would be another place to try. If you know of any Filipino restaurant you could also try asking them.
BT, wakame is similar to alaria, another type of sea vegetable. It's the seaweed that you find in miso soup. It doesn't really have that strong of a flavor in the dish, but you can detect the taste on your tongue. It's not a flavor that stands out, rather a flavor that blends in with the fish.
Yeah, usually we'll pipe out mashed potatoes. Looks better than slopping a spoonful on the plate. The purple potatoes/yams were boiled, then processed through a ricer. It was then mixed with heavy cream and coconut milk to form the mousseline. I actually have a recipe for it here: