Slice the portabello mushroom into long slices, then cut each slice in half.
Melt the butter in a saute pan over medium heat, and saute the mushroom slices until softened and brown.
Prepare the inari pouches according to the directions on the package. (Some require no preparation, while some say you should boil the pouch of inari for a few minutes before using them.)
Mix the mayonnaise and Sriracha sauce until well blended.
Stuff each pouch of inari with about 1/4 cup of rice.
Lay a piece of mushroom on top of each stuffed inari.
Drizzle a little spicy mayonnaise over the mushroom.