Yuzu is a Japanese lemon-like citrus with unique fruit flavor. The condiment,"Yuzu kosho", utilizes the citrus juice and ferments it along with green chilies and salt. It packs a punch so only a small bit is needed but it adds a great citrusy, spicy flavor.
Thinly slice the yellowtail into about 8 equal slices. Arrange on a platter and top each piece of yellowtail with a slice of Serrano pepper, a cilantro leaf and a drop of yuzu kusho. Drizzle the ponzu over the top.