Place cucumber, onion and chile flakes in a mixing bowl. In a small saucepan, over low heat, dissolve the sugar in the water. Remove from the heat and stir in the vinegar and salt. Pour this mixture over the vegetables in the bowl. Stir and refrigerate until served.
2 large cucumbers, peeled, halved lengthwise -- sliced thinly
1 small red onion, peeled, halved -- sliced thinly
1/2 teaspoon dried red chile flakes
4 tablespoons granulated sugar
1/2 cup water
5 tablespoons white vinegar
1/2 teaspoon salt