Stir with a spoon until you have a creamy, smooth sauce.
Taste, then adjust the salt and acidity, adding a little more vinegar if needed.
Add the dill and mint.
Stir to distribute them evenly.
Refrigerate the tzatziki before serving so it's nicely chilled.
Serve chilled, with a few drops of olive oil on top.
Notes
Draining the yogurt is essential. For a large batch and a firmer tzatziki, you can strain it further in cheesecloth hung in a cool spot in the kitchen, for anywhere from 4 hours to overnight; in summer, let it drain in a colander in the refrigerator.Use a very good olive oil, without overdoing the amount: you need just enough to give the mixture a creamy texture.Tzatziki keeps for 3 days in the refrigerator in a tightly closed glass jar. By the next day, it's even more flavorful and aromatic.