Preheat the oven to 200 degrees Celsius (390 Fahrenheit).
Place the salmon fillets on a baking tray, season with salt and pepper, and bake for 15 minutes until thoroughly cooked.
Remove from the oven and allow them to cool.
Place the cubed potatoes in a pot and cover with water.
Add a pinch of salt and boil until tender.
Drain the water, then add milk, salt and butter.
Use a masher to mash until it's light and creamy.
Set aside.
In a saucepan, melt the butter and add the flour, whisking well.
Add the milk and continue to whisk until the sauce thickens.
Next, season with ground black pepper, add the parmesan cheese, dijon mustard, and stir.
In goes the salmon, which can be cut into chunks or use two forks to shred them.
The mixed veggies and chopped spring onion can go in as well.
Give the mixture a good stir and spoon it onto 2 smaller oven-proof dishes or a larger one.
Top it with mashed potatoes and sprinkle over the cheddar cheese.
Bake at 180 degrees Celsius (350 Fahrenheit) for 20-25 minutes until golden and piping hot.