Quick pickle the onions. Thinly slice the red onions and place into a jar. Add the apple cider vinegar, sugar, salt, and water into the jar. Cover the jar, give its quick shake and place it in the fridge for at least an hour.
Make the jicama radish slaw. Thinly slice the jicama into matchsticks, (as close as you can to matchsticks). Thinly slice the radishes using a mandolin. Add the jicama, radishes, and juice of two limes into a bowl, and place in the fridge for at least 30 minutes.
Mix together the sesame oil, soy sauce, and ginger.
Spray a small glass pyrex with Cooking Spray.
Add the cod to the pyrex and pour over the soy mixture.
Place in the oven for 10 minutes. After 10 minutes flip the fish over, and cook for another 10 minutes, until the fish flakes when you separate it with a fork.
Lightly toast the corn tortillas (I did this in my toaster oven).
To assemble, mash some avocado onto a tortilla. Add roughly 1 ounce of fish, a spoonful of the jicama radish slaw, and top with a few slices of pickled onions.