Add the zest of 1 lime to a non-reactive (glass or plastic) bowl.

Then, roll each lime around on a hard surface while applying pressure with your hands so that they'll yield the most juice. Slice limes in half squeeze them out into a measuring cup and set aside.

Prepare your onions onions and peppers by dicing them neatly. Chop up your coriander, and set aside.
  
 
 Step 2:

Take your lovely piece of halibut (or whatever white fish you decide on... tilapia, flounder, cod...whatever) and get your sharpest knife out and cut off about 1/3 of the filet's worth of fish like so.

Cut the fish down again into thin slices. Think sashimi.