Peel thecucumbers, leaving some green stripes, if you like.
Set a large sieve or colander over a bowl. Using a large grater, grate the cucumbers into it.
Squeeze as much juice out of the grated cucumbers as possiblewith your hands, or by wrapping them in a clean kitchen towel and wringing the liquid out, or by using a wooden pestle or a round potato masher to press on the pulp.
Stir the remaining ingredients in with the cucumber.
Cover with plastic wrap and refrigerate, to let the flavors develop, for 1 to 2 hours before serving.
To Serve, stir the tzatziki with a wooden spoon before transferring to a serving bowl. Drizzle with olive oil, and sprinkle with fresh dill or fennel fronds. Serve cold.
Storage
Fridge: Keeps well up to 4 days in an airtight container. Stir well before serving.
Freezer: Note, freezing tends to change the texture of tzatziki, but it can keep up to 2 months in an airtight container. It's a good idea to freeze it in labeled containers, to keep track of the date, in serving sizes only as big as you'll need for one meal. Thaw overnight in the refrigerator, and stir very well to reconstitute before serving.