Finely grate garlic into a medium bowl, add lemon juice and let stand 2 minutes while chopping the herbs.
Add Greek yogurt, lemon zest, Dijon, salt and pepper. Whisk until smooth with no mustard streaks.
Drizzle in olive oil while whisking and continue until the sauce looks glossy and uniform.
Stir in dill, parsley and scallion without blending them into the yogurt.
Taste with the food you plan to serve. Adjust salt first; add a little oil if the yogurt tastes sharply sour.
For dipping, leave thick. For drizzling, whisk in cold water 1 teaspoon at a time until it falls slowly from a spoon.
Serve immediately or refrigerate 15 minutes for the garlic and herbs to distribute.
Stir once before serving and recheck consistency and seasoning.