200g of fish. I used pangasius, even if Samûm says they're the trashcans of the sea. Ask the fish vendor of your choice for good alternatives. Also, this is bound to be delicious with salmon; buy the sushi-quality kind.
1dl lime juice
1dl lemon juice
1/2 red onion, diced
a handful of cherry tomatoes, cut into quarters
salt, pepper, dried chili
later: cilantro and avocado
Please note that lime juice is slightly bitter, more so than lemon juice. You may vary the quantities of either based on that information. Cut the fish into pieces of roughly one centimeter. Together with the other ingredients, put it into a receptacle that can deal with the acid. Ceramic was suggested, but my plastic storage units do just fine. Make sure that the fish is entirely covered with liquid. Let it sit for one hour in a cool place. Drain the liquid from the rest of ingredients. Add the avocado, cut into cubes, and the cilantro for decoration. Serve with toast, or rice, or, indeed, almost any carbon hydrate of your choice.