Preheat your oven to 400°F (200°C). Place the trimmed and washed leeks on a baking sheet, spreading them out in an even layer. Drizzle with a tablespoon of olive oil and gently toss to coat.
Halve the head of garlic horizontally and place it cut-side up alongside the leeks on the baking sheet. Drizzle the garlic with the remaining olive oil.
Roast in the oven for about 35-40 minutes, until the leeks are tender and slightly caramelized, and the garlic is soft and fragrant.
Once roasted, carefully squeeze the softened garlic cloves out of their skins into a large pot, mashing them slightly with a spoon to release their sweetness.
Chop the roasted leeks into smaller pieces and add them to the pot with the garlic.
Pour the vegetable broth into the pot, then bring to a gentle simmer over medium heat. Season with salt and pepper.
Let the mixture simmer for about 10 minutes to allow the flavors to mingle and the leeks to soften further.
Use an immersion blender or transfer the soup to a blender in batches, blending until smooth and velvety in texture.
Taste and adjust salt and pepper as needed. Reheat gently, if necessary, to serve hot.
Serve the soup hot, garnished with a drizzle of olive oil or fresh herbs if desired.
