Wet a clean piece of food-safe cotton fabric such as a butter muslin and wring out any excess water. You can also use a large coffee filter or even a wet paper towel. Line the inside of a wire sieve with the damp cloth or filter and set it over a bowl.
Pour the yogurt into the lined sieve. Cover and refrigerate for 2-3 hours. You'll have a lot of whey accumulated in the bottom of the bowl which you can discard (or save and use to marinate meat or fish). Put the sieve back over the bowl and continue to drain the yogurt overnight.
When the Labneh is done, it should be at least as thick as soft butter, but you can continue straining it longer if you want it to be firmer.
Transfer the Labneh to a serving dish and use a spoon to flatten it out and make swirls in the top.
Drizzle the labneh dip with 2 tablespoons olive oil. Garnish with zaatar and sumac.