Ingredients

    1 1/2 cups plain lowfat Greek yogurt
    1 lb Persian or English cucumbers, peeled and seeded
    3 tbsp extra virgin olive oil
    1 tbsp fresh chopped mint or dill

    1 clove garlic, crushed (or more to taste)
    1 tbsp fresh lemon juice (or more to taste)
    1/4 tsp salt (or more to taste)
    Fresh mint sprig for garnish (optional)

Strain the Greek yogurt for 2-3 hours in the refrigerator. If you'd prefer to use plain yogurt and not Greek, you'll need to strain it for 14-15 hours to get the right consistency. If you don't know how to strain yogurt, you can find instructions here:

Pulse the cucumbers in the food processor or hand chop to desired texture. I like them chopped small, but not minced. Bigger chunks will result in chunkier tzatziki. It’s a matter of preference. You can also grate them, if you prefer.

Cut out an 18-inch rectangle of cheesecloth and fold into two layers. Place chopped cucumbers in the center of the double-layered cloth. Gather up the cheesecloth and twist at the top to form a bundle. Squeeze the bundle several times over the sink, twisting the bundle tightly to get rid of as much liquid as possible. If you don't have cheesecloth, use a thin dishtowel instead.

Place strained yogurt in a medium mixing bowl.

Add the drained cucumber pieces from the cheesecloth to the bowl along with the olive oil, chopped fresh mint or dill, crushed garlic, lemon juice, and salt.

Use a fork or whisk to blend the ingredients together. Taste the tzatziki; add more garlic, lemon juice, or salt to taste if desired. Serve cold. Store in the refrigerator.

http://theshiksa.com/2011/08/24/tzatziki/