Prepare your cucumbers and onions by slicing them thinly using a knife on a cutting board.
In a mixing bowl, combine the sliced cucumbers and onions, add the salt, and toss lightly to ensure even coating. Let them sit for about 30 minutes to draw out excess moisture.
After 30 minutes, drain the cucumbers and onions and set them aside.
In a saucepan over medium heat, combine the apple cider vinegar, granulated sugar, mustard seeds, celery seeds, turmeric, and black pepper. Stir until the sugar has dissolved, and the mixture comes to a simmer.
Add the drained cucumbers and onions to the saucepan, stirring to coat them well in the vinegar mixture.
Allow the mixture to simmer for about 10 minutes, stirring occasionally.
Remove the saucepan from heat and let the pickles cool slightly.
Once cooled, pack the pickles and liquid into sterilized mason jars, leaving about ½ inch of headspace. Seal tightly with lids.
Let the jars cool to room temperature, then refrigerate them for at least 24 hours before enjoying, though they taste best after a few days.