Velvety Carrot Soup with Carrot Top Pesto

Prepare a carrot top pesto to make the most of the fresh carrot flavor.
Heat 1 tablespoon oil in a medium saucepan over medium-high heat. Add the onion and 1/2 tsp salt and cook until softened, stirring occasionally, about 8 minutes.Remove the carrot tops, coarsely chop and set aside. Coarsely chop the carrots and add them to the pan. Pour in the broth and bring to a boil. Turn down the heat, cover (leave the lid slightly ajar) and cook until the carrots are very soft, about 20 minutes.Meanwhile, chop the garlic in a food processor, scraping off the sides of the bowl if necessary. Add 1 cup carrot tops, walnuts, remaining 2 1/2 tablespoons vegetable oil and remaining 1/4 teaspoon salt and beat until smooth.Whisk the carrot-stock mixture in a blender, working in batches, until smooth, about 1 minute per serving. Pour into plates and roll in carrot pesto sauce.