Preheat oven to 350°F (175°C).

Spread potatoes and leeks on parchment paper-lined cookie sheets.

Drizzle vegetables with olive oil and sprinkle with salt, pepper, and thyme.

Cut the top off the head of garlic, drizzle with olive oil, and place on the cookie sheet with the vegetables.

Bake for 60 minutes, or until vegetables are tender.

Place roasted potatoes in a large Dutch oven and mash.

Remove roasted garlic cloves from head and add to the Dutch oven.

Add half of the roasted leeks to the Dutch oven.

Pour in chicken broth and heat over low to medium heat until ingredients come to a simmer.

Puree ingredients with an immersion blender until smooth.

Check seasonings and add salt, pepper, and thyme to taste.

Ladle soup into bowls and top with reserved roasted leeks and fresh thyme leaves.