Let’s get cooking.
Heat the oven to 200°C fan or 220°C conventional. Set out a deep baking dish about 30 × 20 cm. Peel the potatoes and cut into roughly 3 cm chunks. Peel the 2 garlic cloves and leave them whole.
Put the potatoes and whole garlic in a saucepan and cover with cold water. Bring to the boil, then simmer for 15–20 minutes until a knife slides through the potatoes easily. Start the filling while they cook.
Cut the chicken into roughly 2 cm pieces and chop the onion. Trim the leek, split it lengthways and rinse between its layers to remove grit, then slice it. Grate the cheddar.
Heat the 1 tbsp oil in a large deep frying pan over medium-high heat. Fry the lardons for about 4–5 minutes until starting to colour. Add the chicken and cook for about 4–5 minutes, turning, until lightly browned outside.
Add the onion and leek. Reduce to medium heat and cook for 6–8 minutes, stirring, until softened. Stir in the cream, mustard, salt and pepper, then simmer gently for 3–4 minutes until the sauce coats the filling. If still very runny, simmer a little longer.
Drain the cooked potatoes and garlic, return them to the warm saucepan off the heat and leave uncovered for 1–2 minutes so excess moisture steams off. Mash with the 50 g butter and 100 ml milk until smooth.
Spoon the hot chicken filling into the baking dish. Dot the mash over it in spoonfuls, then gently spread to the edges. Roughen the top with a fork and scatter over the 150 g cheddar.
Bake for about 20 minutes until golden on top and bubbling in the centre. Check the centre of a large chicken piece reaches 75°C for 30 seconds. Let the pie stand for 5 minutes before spooning onto plates.
