Place oven rack in top position. Preheat oven to 425°F.
Toss cauliflower with 2 teaspoons olive oil, and season with salt. Spread on large cookie sheet, and bake 12 to 15 minutes, or until cauliflower is fork-tender and slightly browned.
Meanwhile, heat remaining 2 teaspoons olive oil in small saucepan over medium heat. Sauté garlic in oil 1 to 2 minutes, or until fragrant. Stir in tahini, lemon juice, 5 Tbs. water and salt. Simmer over low heat 1 to 2 minutes. Remove from heat.
Divide cauliflower among plates. Whisk sauce, then spoon over cauliflower. Sprinkle with parsley and sesame seeds, and serve.