Simmer the tofu for 15 minutes in salted water. Turn off heat an allow to cool. Drain and leave to dry on kitchen roll.
Heat oil in a wok. Fry tofu until golden. Drain on kitchen roll.
Whisk cream and nam prik pao together in a bowl.
Combine the sugar, soy sauce, seasoning sauce, and water in a another small bowl.
Heat oil in wok or pan. Add onion and saute until soft. Then add garlic, ginger, and curry powder. Fry for a minute. Add tofu and fry for another 30 seconds. Add spring onion, chinese celery, and soy sauce mixture. Cook for another 30 seconds.
Pour milk mixture over and simmer for a a minute or two. Taste and season.
Garnish with sliced chilies, fresh coriander, and sesame seeds (all optional) and serve with steamed jasmine rice.