In a food processor or blender, combine garlic and parsley with the oil, salt, lemon juice and water. Gently puree until the sauce has the consistency of runny yogurt (add more water, if necessary). Taste and adjust salt.
Heat the oven to 450 degrees.
Toss the broccoli and cauliflower with the oil, salt, and pepper. Place on an oiled baking sheet (or two) in an even layer. Roast until the tops are nicely browned, stirring and flipping over (tongs are a good tool for this) after 8minutes, roasting about 15-20 minutes total (until tender).
Remove from the oven and transfer to a platter or to individual serving plates.
Drizzle on the tahini sauce, sprinkle with remaining parsley and serve.