Bring a large saucepan of water with ½ tsp salt to the boil. 

Chop the potatoes into 2cm chunks (peel first if you prefer). 

When boiling, add the potatoes to the water and cook until you can easily slip a knife through, 15-20 mins. 

Meanwhile, halve, peel and chop the onion into small pieces.

Heat a drizzle of oil in a large frying pan on medium-high heat.

Once hot, add the bacon lardons and onion. Stir-fry until golden, 4-5 mins. IMPORTANT: Wash your hands and equipment after handling raw meat. Cook bacon thoroughly.

While the bacon and onion are cooking, peel and grate the garlic (or use a garlic press).

Trim the root and dark green leafy part from the leek and discard. Halve lengthways, then thinly slice.

Trim the carrot, then quarter lengthways (no need to peel). Chop widthways into 1cm pieces.

Once the bacon is cooked, add the leek, carrot and sliced mushrooms to the pan. Season with salt and pepper.

Cook, stirring occasionally, the veg until softened, 7-8 mins. Stir in the garlic and cook for 1 min more. 

Meanwhile, preheat your grill to high.

Once the potatoes are cooked, drain in a colander and return to the pan, off the heat. Add a knob of butter and a splash of milk (if you have any) and mash until smooth. Season with salt and pepper.

Once the veg has softened, stir the butter and flour (see pantry for both amounts) into the pan and cook, stirring, for 1-2 mins.

Add the creme fraiche, wholegrain mustard (add less if you'd prefer), chicken stock paste and water for the sauce (see pantry for amount) and simmer for 3-4 mins. 

Pour the pie filling into an appropriately sized ovenproof dish. Spoon over the mash, using the back of a spoon to smooth it out.

Grill until your pie is golden and bubbling, 3-4 mins.

Share between your serving plates when ready.