Chop pumpkin and carrots, simmer in water to cover, add stock cubes, cook till pumpkin and carrots are soft.
In separate small, heavy skillet, heat oil. When hot, add mustard seeds, cover pan and keep over high heat until the sound of popping dies down a bit, then immediately add the onion and reduce heat.
Cook and stir until onion is clear. Measure the spices while the onion is cooking; then stir them into the mixture and allow to cook on low heat for a minute or so until they are fragrant.
Turn into the pumpkin pot, using a cup or so of cooking water to rinse the spice pan into the soup pot.
Puree the seasoned pumpkin and carrots in their cooking water, adding the coconut milk after mashing.