Preheat your oven to 220°C. Bring a medium saucepan of water to the boil with 1/2 tsp of 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). Roughly chop the parsley (stalks and all). Grate the Cheddar cheese. Chop the cod into 2cm chunks. IMPORTANT: Remember to wash your hands and equipment after handling raw fish.

Once boiling, add the potatoes to the water 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. Cook, stirring constantly, until it forms a paste consistency, 1-2 mins. Gradually add the water (see ingredients for amount), stirring constantly, then add the stock paste. Bring to the boil, stirring continuously then reduce the heat and simmer, stirring, until the mixture thickens and is smooth and creamy, 1-2 mins.

Add the creme fraiche, parsley and the fish, stir, and cook for 3-4 mins. Taste the sauce and season with salt and pepper if necessary. Add the prawns to the mix, cook for 2-3 mins. The prawns should be par-cooked at this point. IMPORTANT: The fish is cooked when it is opaque in the middle. Pour into an appropriately-sized ovenproof dish.

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 and spread it out evenly. Sprinkle over the grated cheese and bake on the top shelf of your oven until golden, 12-15 mins. TIP: Pop the dish on a baking tray to catch any drips as it bakes. IMPORTANT: The prawns are cooked when pink on the outside and opaque in the middle.

Wash out your (now empty) frying pan. Trim the green beans. Heat a drizzle of oil in your frying pan on medium-high heat. Once the oil is hot, add the beans and season with salt and pepper. Stir-fry until starting to char, 3-4 mins. Stir in the garlic and add a splash of water. Cover the pan with a lid or some foil and cook until tender, 2-3 mins. Then remove the pan from the heat.

Spoon the fish pie into bowls or onto plates and serve with the green beans on the side. Enjoy!