1 cup water
1/2 cup distilled white vinegar
1/4 cup vegetable oil
1/4 cup sugar
2 teaspoons salt
1 tablespoon fresh, coarsely ground black pepper
3 cucumbers, peeled and sliced 1/4-inch thick
3 tomatoes, cut into wedges
1 onion, sliced and separated into rings
Whisk water, vinegar, oil, sugar, salt, and pepper together in a large bowl until smooth; add cucumbers, tomatoes, and onion and stir to coat.
Cover bowl with plastic wrap; refrigerate at least 2 hours.