Heat oven to 180°c. Peel, de-seed, and cube the pumpkin. Set the pumpkin cubes is a baking tray, drizzle with some oil, and roast for 20 minutes, or until soft.
Peel and dice the onion. Mince ginger and garlic. Slice chilis. Mince the white part of the lemongrass and save the greenish stalk.
Heat a heavy-bottomed saucepan without oil. Toast coriander, cumin, and fennel seeds are dry pan until fragrant. Transfer toasted spices to mortar and grind them to coarse powder. Add cayenne pepper, turmeric, and pepper, and mix spices together.
Heat some oil in the saucepan. Add onions and fry until sautéed. Add garlic, ginger, half of the chili slices, minced lemongrass, and dry spice mixture. Fry for a couple of minutes. Add half of the vegetable broth, reserved lemongrass stalk and kaffir lime leaves. Simmer the broth for 10 minutes or until butternut pumpkin is done.
Take the saucepan off from the stove. Remove lemongrass stalk from the broth. Add about one-third of the roasted butternut pumpkin cubes to saucepan. Puree soup with an immersion blender. Set the saucepan back to heat and add rest of the broth, lime juice, tamarind paste, soy sauce, brown sugar, and coconut milk. Adjust the taste. Add rest of the chili slices if desired. Add more broth if the soup seems too thick. Cube the tofu and add it, along with the remaining pumpkin, to the soup.
Slice the red onion to thin half moons, chop the scallions, slice the chili, and cut the lime to wedges. Prepare rice noodles according to package instructions. Divide noodles to soup bowls and ladle some soup over them. Garnish with scallion, red onion, chili slices, and lots of fresh coriander. Serve with lime wedges.