Strip the kale leaves from the stems and use a knife to chop them into shreds. Make the same sized shreds with the savoy cabbage. Place the kale, cabbage, parsley, cilantro and chives in a bowl.
In a small bowl, whisk together the mayo, relish, mustard and apple cider vinegar until smooth. Pour the dressing over the slaw and use kitchen tongs (or even your hands) to toss it well.
To assemble the sandwiches, add a bit of tartar sauce on the bottom of each bun. Top with a piece of fish, then with the slaw. Serve immediately!
Note: this slaw actually stays great in the fridge for 1 to 2 days. However, the fish breading is so light, that is really must be eaten right after cooking, or else it gets soggy.