Avocado halves filled with tuna, tomato, red onion and cilantro bound with mayonnaise. No cooking at all, ten minutes, and the lime goes on the avocado as well as in the filling. Serves six.
The avocado is the bowl, so it has to be ripe enough to eat and firm enough to hold together. Brush the cut surfaces with lime as soon as they are exposed: the acid slows the browning, and an avocado left cut on the counter turns grey within twenty minutes. The flesh you scoop out to enlarge the hollow can be chopped and stirred into the filling rather than thrown away. Assemble these shortly before serving, because they do not hold well: even with lime the surfaces darken over a couple of hours.