Preheat the oven to 180°C fan / 200°C / 400°F
Place the chunks of potato, carrot and swede into a saucepan of boiling water and bring to the boil. Cook for 15 - 20 minutes, until the veg is tender. Drain well and set aside.
Measure the milk into a jug and add the chopped spring onion. Set aside for the flavours to infuse.
Whilst the vegetables are cooking, put the flour and low fat spread/light margarine in a saucepan and heat, stirring continuously until the spread has melted and it has combined with the flour to make a thickish paste (roux).
Continue to cook for a further 1 minutes before slowly whisking in the milk & onion bit by bit. Do this slowly to prevent lumps from forming.
Bring to the boil then reduce the heat and cook for a further 3 - 4 minutes until the sauce has thickened slightly.
Turn off the heat and then stir in the mustard and a little salt & pepper.
Add the chunks of fish, the shrimp and the peas and stir till they are all coated with the sauce.
Spoon into a 4 serving casserole dish.
Use a potato masher (or a potato ricer - my preference) to mash the cooked potatoes, swede and carrots into a smooth mash. Season with a little salt and pepper.
Now, you can either pipe the vegetable mash on top of the fish (like I did) or you can distribute spoonfuls of it over the fish base and use a fork to carefully spread it out.
Bake in the preheated oven for 20 - 25 minutes until the vegetable mash has turned a little crispy and golden and the filling is bubbling hot!
Serve with garden peas or sweetcorn.