Pre-heat the oven to 200C/Gas 6. Peel the squash, cut into chunks, drizzle with oil and season with salt and pepper. Cook in the oven until just tender, remove and set aside.
Peel the garlic, shallots and ginger. Remove the tough outer leaves of the lemongrass and discard. De-seed the chillies, if you don’t want too much heat.
Chop everything finely and add to a food processor along with the cumin and half of the coriander. Whizz until everything is very finely chopped, then add the fish sauce and whizz again.
Heat a tablespoon of oil in a pan on a low to medium heat and cook the paste for five or six minutes, stirring occasionally.
Pour in the coconut milk, then add the boiling water. Sprinkle in the bouillon powder and stir. Gently bring to a boil, then reduce to heat and simmer for 10 minutes until slightly reduced and thickened.
Roughly chop the tomatoes and add - cook gently until they are tender but still have some bite, you want them to hold their shape. Gently fold in the chunks of butternut squash and heat through. Stir in the rest of the coriander, roughly chopped, saving a few sprigs for garnish along with a squeeze of lime. Serve with rice