For easy blending, bring the tahini paste to room temperature or warm it slightly in the microwave before adding the lemon juice; don't worry when it seizes up. Adding the water gives the sauce a creamy texture. More Terrific Condiments
In a medium bowl, toss the onion with the cumin and pepper.
In a small bowl, using a whisk, work the lemon juice into the tahini. Gradually whisk in the water until smooth. Season with salt. Stir the sauce into the onion, then stir well and garnish with nigella seeds.
The sauce can be refrigerated overnight. Bring to room temperature before serving.
Köfte with Pistachios and Tahini Sauce