Wash cucumbers thoroughly and slice them into uniform 1/8-inch rounds using a mandoline for consistency. Slice the onion thinly and place both vegetables in a large bowl.
Sprinkle with kosher salt, toss gently, and let stand for 2 hours to draw out excess moisture.
Thoroughly rinse the cucumber-onion mixture under cold water to remove excess salt. Drain completely and pat dry with clean kitchen towels or paper towels.
In a medium saucepan, combine the sugar, white vinegar, apple cider vinegar, mustard seeds, celery seeds, turmeric, cloves, and bay leaves. Bring to a gentle boil over medium heat, stirring occasionally to dissolve the sugar completely.
Allow the brine mixture to simmer for 5 minutes to activate the aromatic compounds in the spices.
Pack the cucumber and onion mixture into clean glass jars. Pour the hot brine over the vegetables, ensuring they're completely submerged.
Let the jars cool completely to room temperature on your countertop.
Cover the jars tightly with lids and refrigerate for at least 24 hours before serving, though 48 hours yields optimal flavor development.