This sauce, rich with watercress and mint, loads vibrant color and flavor into a simple tahini sauce, one of my home staples for quick flavor. The blanching step ensures a bright green color and also makes it easier to blend together the tahini and the greens.
Cook the garlic in a medium saucepan of boiling salted water until slightly softened, about 30 seconds. Using a slotted spoon, transfer the garlic to a bowl of ice water; let cool.
Return the water in the saucepan to a boil and cook the watercress, mint, and parsley just until wilted, about 15 seconds; drain. Transfer the watercress and herbs to ice water; let cool. Drain the watercress, herbs, and garlic and squeeze between paper towels to remove as much moisture as possible; coarsely chop.
Purée the watercress, herbs, garlic, tahini, lemon juice, 1 teaspoon salt, and 3/4 cup water in a blender until smooth, thinning with more water as needed to reach the desired consistency; season with salt and more lemon juice, if desired.
Do ahead: The sauce can be made 3 days ahead. Cover and chill. Shake before using.