I like to experiment with different mashed potato mixes by adding up to 50% carrot, swede, butternut squash and/or sweet potato.
Serve with a green vegetable.
Cut fish into 1" chunks and mix together with the shrimps in a small saucepan with the onion and mushrooms. Just cover with cold water, bring to a boil and simmer gently for 5-7 minutes until fish is just cooked. Take off the heat and mix the sauce mix with the soy milk and gradually add to the fish stirring constantly to prevent lumps forming. Return to the heat and allow the sauce to thicken then pour into an ovenproof dish. Mix the mashed potato and root mash together and layer over the fish. Use a fork to create furrows and wash over with a little egg. Bake in the oven at 180C until the top has browned, about 60 minutes.