Preheat your oven to 220°C. Bring a medium saucepan of water to the boil with 1/2 tsp salt. Peel the potatoes and chop into 2cm chunks. Trim the root and the dark green leafy part from the leek. Halve lengthways, then thinly slice widthways. Peel and grate the garlic (or use a garlic press). Grate the Cheddar cheese. Chop the cod into 2cm chunks. IMPORTANT: Wash your hands and equipment after handling raw fish.

Once your pan of water is boiling, add the potatoes and cook until you can easily slip a knife through, 15-20 mins. Meanwhile, pop a large frying pan on medium-high heat. Add a drizzle of oil and half the butter. Allow the butter to melt, then add the leek. Stir-fry until the leek is soft, 3-4 mins, then stir in the flour (see ingredients for amount) and cook, stirring frequently, until it forms a paste, 1-2 mins. Gradually stir in the water for the sauce (see ingredients for amount), then add the veg stock paste and bring to the boil, stirring continuously. Reduce the heat and simmer, stirring occasionally, until thickened, 1-2 mins.

Once the sauce has thickened, stir through the creme fraiche and the cod chunks. Simmer for 3-4 mins. Taste and season with salt and pepper if needed. Add the prawns and simmer for 2-3 mins until mostly pink, then pour the fish mixture into an appropriately sized ovenproof dish. IMPORTANT: The fish is cooked when opaque in the middle.

Once the potatoes are cooked, drain in a colander and return to the pan, off the heat. Add the remaining butter and a splash of milk (if you have any) and mash until smooth. Season with salt and pepper. Spoon the mash on top of the fish mixture in an even layer. Sprinkle over the grated cheese and bake on the top shelf of your oven until golden, 12-15 mins. TIP: Put the dish onto a baking tray to catch any drips. IMPORTANT: The prawns are cooked when pink on the outside and opaque in the middle.

While the pie bakes, trim the green beans. Wipe out your (now empty) frying pan and pop on medium-high heat with a drizzle of oil. Once the pan is hot, add the green beans and stir-fry until starting to char, 2-3 mins. Stir in the garlic, turn the heat down to medium and cook for 1 min. Then add a splash of water and immediately cover with a lid or some foil. Cook until the beans are tender, 4-5 mins. Remove the pan from the heat.

When everything is ready, spoon the fish pie into bowls or onto plates. Serve with the garlicky green beans alongside. Enjoy!