Saute the garlic and onion in the olive oil in a large saucepan over medium heat for 3 to 4 minutes. Add the carrots, potatoes, and pumpkin and stir to saute. Pour in the water and bring to a boil. Tightly cover the saucepan and simmer until all the ingredients are tender, about 15 minutes.
Put the mustard and coriander seeds in a small saucepan over low heat, cover, and, shaking the saucepan frequently to move the seeds inside without opening the lid, cook until you hear the sound of mustard seeds popping. As you hear the last one pop, remove from the heat and crush the seeds with a mortar and pestle. Sift the spice mixture through a strainer to make sure the husks are removed.
Add 1 1/2 teaspoons of the sifted spice mixture, the grated nutmeg, and pepper and salt to taste to the soup.
Pour in the milk and puree the soup using a hand blender or a food processor.
Pour the soup into bowls and serve right away. Garnish with a dollop of sour cream on top.