Trim and grate zucchini on the wide holes of a box grater. Place zucchini shreds in a colander and toss with ½ tsp salt. Place inside a large bowl (or in the sink), and leave to drain for 30 minutes.After 30 minutes, wrap zucchini in a clean dish towel and squeeze out as much water as possible. Transfer strained zucchini to a large mixing bowl.
Add breadcrumbs, feta, eggs, scallions, dill, cumin, garlic powder, remaining ½ tsp. salt, and black pepper; mix well to combine (I use my hands to thoroughly mash together).Use a ¼ cup measure to scoop up mixture and form into 10 uniform patties (no thicker than ½ inch). Place on a parchment-lined baking sheet and refrigerate for at least 30 minutes.
Heat olive oil in a large skillet over medium. Once shimmering, arrange half of zucchini cakes in skillet. Fry until golden brown, gently turning once with a spatula.Transfer to a plate or drying rack
Combine tzatziki and harissa in a medium bowl; mix well.Serve zucchini cakes with harissa tzatziki and pickled red onion, if desired.
