Stir carrots, potatoes, onion and garlic together in a large roasting pan; drizzle with oil and sprinkle with herbes de Provence, salt and pepper.
Roast in the preheated oven until vegetables are tender and beginning to brown, stirring once, about 50 minutes. Remove roasted vegetables from oven; scrape vegetables and any brown bits into a large pot. Pour vegetable stock into vegetable mixture; bring to the boil.
Blend soup until smooth with an immersion blender; stir in cream (or milk) to thin soup to desired consistency.