Put a medium pan of water on to boil over a high heat. Peel the potatoes and chop them into small, equal-sized chunks. When the water is boiling, add the potatoes. Simmer for 20-25 mins till soft, then drain. Set aside.
While the potatoes cook, peel and finely slice the shallots. Trim the roots off the spring onions and finely slice them. Warm a pan for 1 min, then add the shallots, spring onions and 1 tbsp oil. Season with salt and pepper. Cook over a medium heat for 5 mins, stirring once or twice, till the veg are soft.
While the veg cook, coarsely grate the cheddar cheese. Roughly chop the lovage leaves. Heat your grill to high.
Stir 1 tbsp cornflour into the veg. Cook and stir for 2 mins. Slowly add 400ml milk to the pan in little splashes, stirring well as you add it, till it's all combined and smooth. Turn the heat up and bring to a simmer, then simmer for 8-10 mins till the sauce has thickened a little. Stir now and then.
Stir the fish pie mix into the sauce. Simmer for 5 mins, stirring once or twice, till the fish is cooked. Stir in half the cheddar, all the spinach and most of the lovage.
Pour the fish pie filling into a snug ovenproof dish. Mash the potatoes with the remaining milk and some salt and pepper. Spoon over the fish pie. Scatter over the remaining cheese.
Grill the fish pie for 15 mins till golden and bubbling. Let it rest for a few mins, then serve sprinkled with the last of the lovage.