Position oven rack in center of oven and preheat oven to 400°F. Scrub potatoes well. Do not peel. Prick each potato several times with the tines of a fork. Place potatoes directly on oven rack. Roast until fork easily inserts into center of potato without resistance, approximately 45 minutes - 1 hour, depending on size of potatoes. Begin checking for doneness around the 40-minute point. Cool potatoes just enough to handle to remove the flesh from the skins.

Line a small rimmed baking sheet with a piece of tin foil large enough to wrap up the garlic head. Lightly spray tin foil with cooking oil. Peel off outer papery skin of garlic. Do not separate the cloves; leave them intact. Slice off about 1/3” from the top of the garlic head and discard. Place the garlic head upright on the prepared baking sheet and drizzle 1½ - 2 teaspoons of olive oil all over the exposed cloves. If desired, season with a sprinkle of fine sea salt and freshly ground pepper. Gather up ends of foil to form a packet covering the garlic head. Roast at 400°F for 35-45 minutes, or until garlic cloves are caramelized and golden brown and cloves show signs of pulling away from their skins. When cooled enough to handle, squeeze the garlic cloves from their skins and mash with fork. Set aside.

When potatoes have cooled enough to handle, remove the pulp from the skins and mash well with a potato/vegetable masher.

While the garlic and potatoes are roasting, cut off and discard the dark green part of the leek as well as the stem end. Wash leek well. Chop the leek (white and light green parts only) and dice and measure onion, carrots, and celery. Grate the Parmesan cheese. Set out the apple to be peeled, cored, and diced for when called for in the recipe. Measure out all remaining ingredients except the cream.

Heat olive oil in large skillet or sauté pan over medium heat. When pan is hot and the oil is starting to shimmer, add the butter. Once the butter has melted, reduce heat slightly and add the celery and carrots. Cook, stirring frequently, for a couple of minutes then add the onions. Cook for about 2 minutes longer then reduce the heat to medium low and add the leeks. Cook, stirring frequently, for about 4 minutes then add the peeled, cored, and diced apple and cook for a couple of minutes longer.

Transfer mixture to large soup pot. Place over medium heat and add the warm poultry stock, salt, pepper, and seasonings. Bring mixture just to the boiling point then cover and reduce heat to simmer for about 15-20 minutes. Stir in the mashed sweet potato and roasted garlic and cook, uncovered, over medium-low heat for approximately 5-7 minutes, stirring occasionally.

Bring cream to room temperature.

Cool soup mixture slightly then purée using either a hand-held immersion blender, a traditional blender, or food processor. If using a traditional blender or food processor, purée in batches.

Ladle soup into bowls and, if desired, garnish with a dollop or swirl of sour cream and a sprinkle of pumpkin or sunflower seeds. Serve with rolls, bread (plain or garlic), focaccia, biscuits, or savory crackers.