Slice cucumber in half lengthwise. Scoop out seeds. Grate cucumber with a box grater. Add cucumber to a clean cloth, or a colander, and squeeze to remove as much liquid as possible.
In a medium bowl, mix together everything except olive oil.
Garnish with olive oil and dill if desired.
Notes
Tzatziki can be stored in an airtight container in the fridge for up to 3 days. Stir before serving.