If yogurt is not very thick, strain through cheesecloth over a bowl in the fridge for 2+ hours.
Peel and seed the cucumber. Grate on the large holes of a box grater.
Salt the grated cucumber, let sit 10 minutes, then squeeze dry in a towel.
Grate or mash garlic to a smooth paste.
Combine strained yogurt, cucumber, garlic, olive oil, and vinegar in a bowl. Stir well.
Season to taste with salt and pepper. Fold in dill if using.
Cover and refrigerate at least 2 hours (overnight is best).
Stir, transfer to serving bowl, drizzle with olive oil, and serve.