In food processor, pulse 1 pita quarter, garlic, cayenne, and cumin until blended. Grill bell pepper over medium-high heat until charred in spots, about 5 minutes. Peel and coarsely chop bell pepper. Add bell pepper, walnuts, oil, and lemon juice to processor bowl; pulse until almost smooth. Transfer to wide shallow bowl. Garnish dip with cilantro. Serve with remaining pita quarters.