This is basically strained yogurt, flavored with dill, salt, garlic and cucumbers. It's good on gyros and great with falafel (hopefully I'll post a falafel recipe tonight!)
Using the yogurt recipe, make half a gallon of yogurt and let it set for ~7 hours. Line a colander with fine cheesecloth (use multiple layers) and place some sort of drainage pan to catch the whey under the colander. Pour the set yogurt into the colander and allow to drain overnight.
Once the whey has drained (about a quart will drain from a half gallon of yogurt) finely chop some dill and a few garlic cloves (I'd use a lot, maybe 4-5, but I love garlic a lot, do this to taste). Set the dill and garlic in a bowl together with a sprinkle of sea salt (maybe 1 tsp). Peel and chop a cucumber (remove the seeds) and mix with a little salt to dry it out. Pour off the cucumber liquid.
Put the yogurt into a food processor, pulse once and add the other ingredients. Pulse until smooth and uniform. This will keep a week or two in the fridge (I use an old clean yogurt container to store it).