In a bowl, whisk together Greek yogurt, Kewpie mayo, lemon juice, garlic, onion powder, sumac, turmeric, salt, and black pepper. Chill until ready to serve.
¾ cup Greek yogurt, ½ cup Kewpie mayo, 1 tablespoon lemon juice, 1 garlic clove, ½ teaspoon onion powder, ¼ teaspoon sumac, ¼ teaspoon turmeric, ¼ teaspoon salt or salt to taste, ¼ teaspoon black pepper