Season fish head pieces with marinade (A) for 25-30 minutes. Just before deep-frying, coat the pieces with beaten egg and toss in cornflour.
Deep-fry in hot oil until golden brown and crispy. Remove and drain well. In the remaining oil in the wok deep-fry the beancurd cubes and yam slices until golden.
In a claypot, heat oil and sesame oil. Fry ginger until fragrant. Add mushrooms and carrot. Pour in combined sauce ingredients (B) and bring to the boil.
Add fish head, beancurd and yam, then reduce the heat and cover the claypot. Simmer for 7-8 minutes. Add spring onion and thicken the sauce with sweet potato mixture.