Preheat the oven to 425 degrees F (220 degrees C).
Mix sweet potatoes, carrots, apples, and onion together in a large baking dish. Drizzle with oil and toss to coat. Stir in curry powder, garlic salt, and pepper. Cover with foil.
Roast in the preheated oven for 20 minutes. Remove foil and roast until vegetables are tender, another 10 minutes. Remove from the oven and allow to cool, about 10 minutes.
Transfer vegetables to a pot and mash or puree with an immersion blender. Stir in milk, ginger, and lemon juice. Bring to a simmer and cook for 20 minutes.