Melt butter in a large frying pan over medium heat. Add ginger, garlic and onion. Cook until garlic begins to brown, about 5 min.
Add squash, coconut milk and green beans to frying pan. Bring to the boil over high heat, then reduce heat to medium, cover and simmer for 30 minutes until squash is tender, stirring occasionally. Stir in prawns and tofu, then season to taste with salt, pepper and sugar.
If acorn squash is unavailable, substitute in butternut squash instead.