Finely slice 1 rasher of bacon and place in a large roasting tray (30cm x 35cm) with 1 teaspoon of oil, then put on a medium heat (starting from cold helps the fat to render out of the bacon).
Wash and trim 1 large carrot and 1 bunch of spring onions, chop into 1cm dice, and once the bacon is lightly golden, stir them into the tray. Finely slice and stir in the stalks from ½ a bunch of fresh flat-leaf parsley. Cook for 10 minutes, to soften, stirring regularly, then stir in 2 tablespoons of plain flour.
Gradually pour in 600ml of semi-skimmed milk, bring up to a good simmer, then stir in 2 teaspoons English mustard and 400g of frozen chopped spinach. Simmer for 5 to 10 minutes, or until the spinach has thawed.
Chop 4 salmon fillets into bite-sized chunks, halve 200g of raw peeled prawns, finely chop the parsley leaves and stir it all through the sauce with 400g of frozen peas, then crumble 125g of feta over the top and turn the heat off.
Meanwhile, wash 600g each of sweet potatoes and spuds (leaving the skins on for extra nutritional benefit) and chop into 3cm chunks. Cook in a large pan of boiling salted water for 15 minutes, or until cooked through, then drain and mash well, taste, and season to perfection.
Gently spoon the mash over the fish pie filling, spreading it out with a fork. Bake for 40 minutes, or until golden and crisp on top, and bubbling at the edges.