In a food processor, blend the tahini, garlic, salt, and lemon juice until the mixture thickens and lightens. Add the water and cumin and blend until smooth. Divide this sauce into two bowls, (about 1 cup each.) To one of the bowls add the chopped parsley. To the other bowl, add the yogurt. Place the florets on a dinner plate and top with the parsley tahini. Sprinkle the top with radishes.
Serve roasted vegetables drizzled with the yogurt tahini.