Preheat the oven to fan-assisted 180C / 200C / 400F / gas 6.
Add the potatoes into some salted boiling water, bring back to the boil, reduce to a simmer cook until tender.
When done, drain, let cool for a few mins and mash with a 4 tbsp of milk and 2 tbsp of dairy-free butter and set aside.
Melt butter in a sauce pan, add green onions and cook for a min, then add in cornstarch and cook for a min.
Slowly whisk in the milk, then bring to a rolling simmer while stirring continuously (to avoid any lumps) for 3 - 4 mins until the sauce is thick.
Switch off the heat, then stir in mustard, nutmeg, anchovies, fish pie mix, baby spinach and peas before transferring mix to a baking dish.
Top with the mashed potatoes and bake for 25 mins in the hot oven until golden and the mixture is bubbling.