Grate the unpeeled cucumber, sprinkle it with the salt, and squeeze out as much of the excess liquid as possible using a clean tea towel or paper towels.
Add the Greek yogurt, minced garlic, olive oil and lemon juice in a small bowl.
Stir the squeezed cucumber into the mixture.
Taste and adjust the salt or lemon juice if needed.
You can serve it immediately, but if you have time, chill it in the fridge for 30 minutes to let the flavors meld.
Notes
Squeeze harder than seems necessary. The drier the cucumber is before mixing, the thicker the tzatziki remains during storage.
Choose thick yoghurt. Thin yoghurt plus cucumber moisture is the most common reason homemade tzatziki turns runny.
Mince garlic finely. Large pieces can taste unexpectedly harsh in a cold sauce.
Adjust after mixing. Yoghurt brands differ in acidity and saltiness, so fixed seasoning isn't always useful.
Give garlic time before adding more. Its flavour spreads through the yoghurt during refrigeration and can taste stronger later.
Use fresh lemon juice. Its clean acidity works particularly well in a sauce with so few ingredients.
Keep it cold. Tzatziki tastes fresher and maintains a better consistency when served well chilled.