Use a mandoline to thinly slice the cucumber and red onion.
Add the cucumber slices to a colander and sprinkle with 1/2 of the garlic salt. Toss to coat then let sit for 15-20 minutes.
In a medium size mixing bowl whisk together the vinegar, sugar and remaining garlic salt. Add 1 Tbsp coarsely chopped fresh dill. (eyeball the amount) Toss to coat then set aside to marinate while the cucumber drains.
After 15-20 minutes, drain the cucumbers then add to the onion mixture, stir to combine.
Serve immediately garnished with freshly ground black pepper and additional dill or store in an airtight container in the fridge until serving.
Notes
Cucumbers: English cucumbers have thinner skin and there's no need to remove the seeds, hence the reason I leave the peeling intact. You could also use garden cucumbers or Persian cucumbers, which are the cucumbers most commonly found in the produce department of most grocery stores.
Onion: You could use thinly sliced green onions, white onion or yellow onion in place of red onions.
Vinegar: You can use red wine vinegar, white wine vinegar, distilled white vinegar or white balsamic vinegar in place of apple cider vinegar.
Sugar: You can omit the granulated sugar and use honey, agave syrup or an artificial sweetener i.e. Truvia or similar that you enjoy.
Spice: You could use crushed red pepper flakes for a hint of spice.
Tomatoes: You could add 1/2 cup diced tomato or halved cherry tomatoes to cucumber salad.
FreshHerbs: You could use fresh chives, tarragon, fresh basil or parsley in place of dill.