To make Pesto: Mince garlic in food processor. Add carrot tops, cilantro, and 1/3 cup toasted walnuts; process until finely chopped. Add Parmesan, and process to combine. Season with salt and pepper, if desired, and pulse in 1 tsp. vinegar. Gradually pour in 1/3 cup oil through tube with motor running; process until incorporated. (Pesto will be thick.)Transfer half of Pesto to small bowl; reserve other half for another use. Whisk remaining 1 Tbs. oil, remaining 1/2 tsp. vinegar, and 1 Tbs. very hot water into Pesto in bowl to achieve dressing consistency; set aside.To make Roasted Vegetables: Preheat oven to 450°F. Combine carrots, tomatoes, onion, and oil in large bowl, and season with salt and pepper, if desired. Transfer to baking sheets. Roast 30 to 35 minutes, or until carrots are browned and tender and tomatoes are shriveled and dark around edges.Use thin spatula to loosen vegetables from baking sheets without breaking them up, and transfer to platter. Dollop Pesto over Roasted Vegetables, top with remaining 1 Tbs. chopped walnuts, and garnish with carrot top sprigs.