Peel and dice the kumara, add to a large saucepan and cover with water. Boil until the kumara is soft. Drain the water and mash with the milk and butter, season with salt and pepper
Add the butter to a large saucepan and melt over a low heat, add the flour and combine with the butter, cook for 1-2 minutes
Remove the saucepan from the heat and add the milk, whisk quickly, and return to the heat. Continue to cook until the sauce thickens, stir often
Add the vegetable stock powder, onion powder, garlic powder, dried parsley and mustard powder to the sauce and stir to combine
Slice the onion and crush the garlic cloves, add to the sauce, and stir to combine
Drain the tins of smoked fish and add to the sauce, mix with a fork to combine
Add the frozen mixed vegetables to a large casserole dish. Add the sauce and mix to combine
Top with the mashed kumara and fluff up with a fork
Cook uncovered in the oven for 30 minutes
Remove from the oven and sprinkle with dried parsley. Allow to settle for 5-10 minutes and then serve