Rinse your ahi under cold water. Now, with a good, sharp knife, slice the tuna into roughly quarter-inch cubes. Typically I butterfly the ahi (cutting it in half horizontally so as to have two thinner pieces rather than the one thick tuna steak), then cut those two halves into quarter inch strips, then cubing it all up. Take these and put into a bowl, and place back in the fridge (to keep it good and cold).
Now take the cucumber you've peeled and seeded (a trick for seeding - take a spoon and drag it against the seeds, as you would, say, a cantelope), and julienne it into quarter-inch strips. These you will then cube into roughly quarter inch pieces. Add these to the bowl that has your ahi in it.
Take your scallions and thinly slice them - specifically, the white part up through the green part before it starts getting leafy - so that you have little, thin, round pieces of scallion. Add these to the bowl that has your ahi and cucumber in it, and keep the bowl out of the fridge for the time being.
Now it's time to add the flavoring - add your finley diced ginger root, sesame oil, soy, teriyaki sauce, sambal olek (or other chile paste), salt, and pepper to the bowl, and, with a spoon, lightly mix it all together, so that the ahi and tuna and scallions are well integrated, and that the mixture of oil and sauce and ginger and paste are well mixed and have coated everything. The reason to do this "lightly" is so that you don't mush up the ahi. Gently, gently...
Now pop it back in the fridge to marinate for around 3 hours or so. I tend to give it a taste about an hour or so in, so as to see how the flavors are maturing together. Not spicey enough for you? Add a little more of your sambal olek / chile paste. Not sesame-y enough? Add a little more sesame oil.
When you're getting ready to serve, take your handful of watercress (that you've rinsed and dried), and add that to the bowl that has your poke in it, and, again, gently, with your hands toss the watercress in with the poke, so that all the leaves become integrated and coated with the oils and flavors.
To serve, take a handful of the poke/watercress, and place it in the center of a plate, and repeat for all four plates. Take what's left in the bowl and equally distribute to the four plates, and with your fingers, elevate the poke vertically, as you would if you were drizzling wet sand on the beach (it should do this fairly easily with the oils and watercress and foundation of the ahi and cucumbers). We're not talking a skyscraper here, just a nice presentation. Now take and sprinkle a little of your toasted ground macademia nuts over your mound of poke, and around the edges of the plate, and then a dusting of red pepper flakes around the edges of the plate, and you're ready to serve!
If you have a nice set of chopsticks, this is a perfect opportunity to break them out.
Enjoy!