Rinse the pumpkin, trim both ends from the gourd and discard the stem and base. Cut the pumpkin in half, spoon out the seeds and stringy innards to discard.
Cut each half of the pumpkin into four wedges, using a vegetable peeler or sharp paring knife remove the pumpkin skin and discard.
Once the gourd is trimmed up, cut each quarter of pumpkin flesh into 1 to 2-inch cubes, and reserve to the side until needed.
Using a food processor or a blender, add the onion and tomato. Pulse a few times until the mixture is well combined and the onions are finely chopped.
In a large skillet, over medium-high heat, add the oil, chili flakes, cumin, curry, mustard, coriander, and salt. Cook the spices for 30 seconds before adding the pumpkin.
Cook the pumpkin for about 5-6 minutes, stirring frequently, until it starts to become a bit tender. Add garlic and ginger, cook for about 1 minute, until fragrant. Add the pureed onion and tomato to the skillet.
Prepare lemongrass by first removing about 1/2 inch of the root end, as well as removing the leaf end of the plant, and discard. Remove a few of the tough outer layers from the remaining stalk, until you have a nice white and light green portion of lemongrass. Smash the lemongrass with the flat side of a meat mallet, or other hard object, to loosen the fibers. Chop horizontally into small pieces.
Add the lemongrass, broth, and coconut milk. Stir to combine, bring to a boil then simmer without a lid for 10-15 minutes.
As soon as the pumpkin is soft and the sauce has thickened remove from the heat, spoon into bowls, and garnish with cilantro.