Prepare the vegetables:
Wash and thinly slice the cucumbers and onions. Place them in a large non-metal bowl.
Mix the brine:
In a separate bowl, whisk together vinegar, sugar, salt, and pepper until the sugar is fully dissolved.
Combine:
Pour the vinegar mixture over the cucumbers and onions. Stir gently to coat everything evenly.
Marinate:
Cover the bowl and refrigerate for at least 2 hours, or overnight for deeper flavor.
Serve:
Serve chilled as a light, refreshing side dish. Store leftovers in a sealed glass jar or airtight container for up to 10 days.