In a 425° F oven, place carrots, apples and celery root in a single layer on a large rimmed baking sheet.

Drizzle with 2 tablespoons of olive oil and toss to coat.

Roast in the preheated oven for 25 minutes, turning once midway through cooking. (The vegetables should be fairly tender and just starting to caramelize when you remove them from the oven).

In a stockpot or large Dutch oven, warm the remaining tablespoon of olive oil.

Add the onion and saute over medium high heat until it begins to soften and turn translucent, about five minutes.

Add the garlic and ginger and saute until fragrant, about one minute.

Add the roasted carrots, apple and celery root, and stir to coat with the onions and spices.

Pour in the vegetable stock, raise the heat and bring to a boil.

Reduce the heat to low and simmer, partially covered, for 15 minutes, or until the vegetables are tender.

Using a regular or immersion blender, carefully puree the soup.

If desired, add the butter to the soup and stir until it is melted and evenly incorporated.

Season to taste with salt and freshly ground black pepper.