Once the mixture cooled down add it to a blender or mash it until you get a thick and chunky sauce, leave some texture.
Keep in mind that the filling should be quite thick, otherwise it will spill out of the onigiri.
Assembling
You can make them by hand by wetting your hand and taking some rice, making a well to put the filling and then closing it to encase the filling within the rice and shaping the ball into a triangle.
But to make things a bit easier I used a triangle mold so I could pieces of coloured rice piece by piece. When I used half of the rice I added a teaspoon of the filling and topped it with the other half of the rice, now press it down tightly so the rice will stick together.
Enjoy.