Position a rack in the center of the oven and heat the oven to 400ºF. Line a heavy-duty rimmed baking sheet with parchment.
In a mortar and pestle, pound the oil, garlic, coriander seeds, fennel seeds, and sage until they resemble a coarse paste. Rub the spice mixture on the flesh of the squash halves. Set them cut side down on the prepared pan and roast until tender when pierced with a fork, about 1 hour. Let cool, cut side up. When cool enough to handle, scrape the flesh away from the rind—you’ll need about 5 cups.
Melt the butter in a 5- to 6-quart Dutch oven over medium heat. Add the leek, carrots, and a big pinch of salt and cook, stirring occasionally, until the leek is softened, 8 to 10 minutes. Add the squash, broth, bay leaf, and 1 tsp. salt and bring to a boil over high heat. Reduce the heat to a low simmer, cover, and cook for 30 minutes to develop the soup’s flavor.
Remove the bay leaf and allow the soup to cool slightly. Purée the soup in batches in a blender. Return the soup to the pot and add the lemon juice. Season to taste with salt and pepper. Garnish with the chopped hazelnuts, chives, and Espelette pepper or cayenne.
nutrition information (per serving):
sat fat g
Photo: Scott Phillips