Position a rack in the center of the oven and preheat to 375ºF. Slice off the top of the garlic to expose the cloves. Drizzle with olive oil, salt, and pepper. Wrap garlic in foil, place on a baking sheet, and roast for 40-45 minutes or until tender. Let cool before squeezing.

While the garlic is roasting, bring a large pot of water to boil. Add peeled potatoes and boil until tender, about 15 minutes. Drain, cool, and cube.

Melt butter in a large dutch oven over medium-high heat. Add onions and leeks and sauté for 6-8 minutes or until they soften and turn translucent.

Lower heat to medium, add flour, and cook for 60-90 seconds until lightly golden. Add 4 cups broth, milk, cream, thyme, ¼ teaspoon salt, and pepper. Let soup boil, then simmer for 2-3 minutes until it starts to thicken. Add potatoes, squeeze in roasted garlic, and parmesan cheese.

Blend the soup until smooth using an immersion blender or in a traditional blender in batches. If thick, add more chicken broth as needed, up to 1 cup. Taste and adjust seasonings.

Serve in bowls topped with parmesan cheese, chives, cracked pepper, or Gruyère grilled cheese croutons.