Preparation

Make the pesto

• Pulse the pepitas in a food processor or blender until finely chopped. Add the basil, cheese, and a pinch of salt, and pulse to combine. With the motor running, add the oil in a slow, steady stream until the pesto is blended; it will be thick.

Make the soup

• Heat the oil in a 4- to 5-quart Dutch oven or other heavy-duty pot over medium-low heat. Add the leeks, celery, and 1/2 tsp. salt, and cook, stirring often, until the vegetables are tender but not browned, 8 to 10 minutes. Add the parsnips, and stir to coat. Add the broth and bring to a boil over medium-high heat. Reduce the heat to low and cook, simmering, until the vegetables are tender, about 20 minutes.
• Working in batches if necessary, purée the soup in a blender until smooth. Wipe out the pot, return the soup to the pot, and season to taste with salt. Stir the pesto, dollop some onto each serving of soup, and sprinkle with a few pepitas, if you like.

Make Ahead Tips

The pesto will keep for 3 days covered and refrigerated.