1. Remove outer papery skins from garlic; cut top quarters off heads and discard. Wrap garlic in foil and roast in 350-degree oven until browned and very tender, about 1 hour. Meanwhile, heat olive oil and 2 thinly sliced garlic cloves in small skillet over medium-low heat. Cook, stirring occasionally, until golden brown, about 15 minutes. Using slotted spoon, transfer garlic slices to paper towel-lined plate and set aside; reserve oil. Once roasted garlic is cool, squeeze cloves from their skins (you should have about 1/4 cup).

2. Combine lemon juice and water in small bowl or measuring cup. Whisk together tahini and garlic cooking oil in second small bowl or measuring cup.

3. Process chickpeas, roasted garlic purée, salt, and cayenne in food processor until almost fully ground, about 15 seconds. Scrape down bowl with rubber spatula. With machine running, add lemon juice-water mixture in steady stream through feed tube. Scrape down bowl and continue to process for 1 minute. With machine running, add oil-tahini mixture in steady stream through feed tube; continue to process until hummus is smooth and creamy, about 15 seconds, scraping down bowl as needed.

4. Transfer hummus to serving bowl, sprinkle toasted garlic slices and parsley over surface, cover with plastic wrap, and let stand until flavors meld, at least 30 minutes. Drizzle with olive oil and serve.

Makes about 2 cups