Wash the cucumber and peel it if necessary. If the cucumber is fresh and undamaged, I don't usually peel it. Grate it using the largest holes on the cheese grater. Place it in a bowl, salt it thoroughly, and let it stand for 10 minutes.
Squeeze out any excess moisture. This step is very important to ensure that the tzatziki is not too runny.
Crumble the feta cheese into a mixing bowl. You can use a fork to break it up if it's too chunky. Add the kefir and yogurt. Season with salt, pepper, olive oil, lemon juice, and crushed garlic. Mix everything together.
Leave the tzatziki in the refrigerator for at least an hour to allow the flavors to blend. It is even better if you leave it overnight.
Serving: Serve the tzatziki cold, garnished with a little olive oil and fresh dill.