Steam the onion, garlic, ginger and celery in oil. Add salt, curry paste and cumin and stir well so that the vegetables are completely moistened with spices. Stir in the pumpkin and lentils, pour in the vegetable stock and bring to the boil.
Simmer over low heat for 15 minutes, then add the tofu. Simmer for another 5 minutes until the lentils and pumpkin are cooked through.