In a large mixing bowl, combine the cucumber slices and onion slices.
Sprinkle salt over the cucumber and onion mixture, toss gently and let it sit for 30 minutes to draw out moisture.
After 30 minutes, drain the cucumbers and onions and rinse under cold water.
In a saucepan over medium heat, combine the sugar, apple cider vinegar, mustard seeds, celery seed, turmeric, and ground black pepper. Bring to a boil, stirring occasionally until the sugar completely dissolves.
Once boiling, add the drained cucumbers and onions to the saucepan. Stir to coat evenly in the brine.
Remove the saucepan from heat and let it cool to room temperature.
Transfer the mixture to a heat-resistant jar and pour the brine over the top, ensuring all the cucumbers and onions are submerged.
Seal the jar and refrigerate for at least 24 hours before consuming to allow flavors to develop.
For best flavor, allow the pickles to sit in the refrigerator for up to a week before using.
These pickles can be stored in the refrigerator for up to 2 months.