Prepare the vegetables: Thinly slice the cucumbers and onions. Place them in a large bowl.
Make the vinegar mixture: In a separate bowl, whisk together the vinegar, water, sugar, salt, and pepper until the sugar is fully dissolved.
Combine: Pour the vinegar mixture over the cucumbers and onions. Stir everything to ensure the vegetables are fully coated.
Chill: Cover the bowl with plastic wrap or transfer the mixture to an airtight container. Let it marinate in the fridge for at least 1 hour (or overnight for best flavor).
Serve: Garnish with fresh dill if desired. Serve chilled as a refreshing side dish or topping for burgers, sandwiches, or salads.