
Prepare all ingredients: peel and cut pumpkin and apple (if using) into chunks, slice onion and mince garlic. Peel off the dry outer layer of lemongrass, cut off the green part, bruise/slightly smash the white part and cut into 3-inch sections.
Place a pot over medium heat, add olive oil. Add onion and sauté until fragrant, then add lemongrass and minced garlic and sauté for a few more seconds until fragrant. If using an electric pressure cooker, you can sauté the aromatics in the inner pot with Sauté function.
Add water, pumpkin and salt. If cooking on the stovetop, bring everything to a boil and then lower to a simmer, cover and cook until pumpkin is very tender (around 10-15 minutes). If using a pressure cooker, cook on High pressure for 3 minutes and then release pressure immediately.
Discard the lemongrass sections. Blend until smooth with a regular blender or stick blender.
Put the soup back on a very gentle simmer, add soy sauce. Then stir in coconut milk. Taste and adjust seasonings and thickness of the soup to your liking.
Transfer soup to serving bowls. Sprinkle with a lot of black pepper. Serve hot with crusty bread and some lime wedges. Garnish with cilantro if desired.