The mousse is simply made with three ingredients: avocados, heavy cream and lemon juice. Give it all a whirl in the food processor but be sure to add the cream slowly with the processor on. Mixing fat and fat can be a bit disastrous otherwise. Next season with salt, cover and store in the fridge until it's time. For the ahi, heat a pan to medium high. Drizzle the ahi with olive oil and give it a rub to coat both sides. Sprinkle sesame seeds, salt and a generous amount of black pepper on both sides. The olive oil should help everything stick on nicely. All it needs now is about a minute on each side to get a nice sear and to leave the middle beautifully rare. Set it aside and have yourself a glass of wine for all that hard work.
Once your guests begin to arrive, use a very sharp knife to cut your ahi steak into thin slices. Plating is easy as pie. One dollop of avocado mousse onto your tray/plate. I like to do the dollop and then smear it with the back of the spoon for a little flair. Place the slice of ahi on top and drizzle with grapefruit oil, lemon zest and fleur de sel.