Heat oil in the Instant Pot using the sauté setting.

Add the chopped onion, garlic, and ginger. Cook while stirring frequently until the onions soften and begin to brown slightly at the edges, about 4-5 minutes.

Stir in the curry paste and continue to sauté for an additional 2 minutes or until the paste becomes fragrant.

Add the butternut squash pieces, bouillon cube, 5 cups of water, and the broken lemongrass stalk along with 1/2 teaspoon of salt. Close the lid of the Instant Pot and set the vent to 'sealing.' Cook at high pressure for 8 minutes.

Perform a quick release of the pressure, open the lid, and remove the lemongrass stalk. Stir in the coconut milk and lime zest.

Using an immersion blender, puree the soup until smooth. If you don’t have an immersion blender, carefully blend the soup in batches using a stand mixer or blender, then return it to the pot.

Mix in the lime juice, black pepper, and sugar, stirring well to dissolve the sugar. Taste and adjust the seasonings as needed.

Serve the hot soup in bowls, garnishing with toppings of your choice such as black pepper, toasted pumpkin seeds, scallions, cilantro, red onion, or red radish.