Preheat oven to 400°F.

Toss the carrots, fennel, and onion with the olive oil. Place on a rimmed baking sheet and roast at 400°F for 20 minutes.

Remove from oven and shake to release any vegetables that may have gotten stuck, then add the garlic cloves. Return to oven and continue roasting for another 20 minutes.

Transfer the vegetables to a 6-quart soup pot, add the potatoes and vegetable stock, and bring to a boil over a medium-high flame.

Reduce heat to low and simmer until the potatoes are tender, approximately 15 minutes. Remove from heat and purée the soup using an immersion blender.

Season with salt and pepper, garnish with pepitas (if using), and serve.