A take-out place near where I work makes an "avocado boat".

A whole avocado is peeled, pitted and cut in half. One half is placed on the bottom of a tray and filled with brown rice and assorted sashimi (raw fish). Then the other avocado half is placed on top. (Perhaps, it should be called a "sandwich"?)

In terms of quantity, there's about as much sashimi as avocado. The rice is clearly minimal. I assume, though that I should make mezonot on the rice and shehakol on the fish. Do I need to make any bracha on the avocado despite its significant quantity? It seems like it is a "tafel" as it is meant to hold the fish, really, and make the dish look pretty, though I am eating the avocado, also.