Slice cucumber crosswise into thin rounds. Combine cucumber and garlic in a medium bowl. Combine vinegar, water, sugar, and salt; microwave at HIGH for 2 minutes or until boiling. Pour hot vinegar mixture over cucumber mixture; stir in sambal oelek. Cover and chill at least 4 hours. Serve with a slotted spoon, or drain before serving. Garnish with sliced green onions, if desired.