Place the bacon into a cold, deep-sided pan and set over a medium-low heat. Allow the fat to render out slowly, then increase the heat and cook until the bacon has good colour and there is fond on the bottom of the pan. Remove and set aside, leaving the fat behind.

Add a knob of butter to the same pan along with the leeks, onion, and garlic. Season with flaked sea salt and white pepper and cook gently until completely soft and sweet. If you are adding any extra greens, stir them in now.

Deglaze the pan with the wine or vermouth, scraping up all the fond from the bottom of the pan. Add a little more butter followed by a few tablespoons of plain flour, stirring to form a roux. Gradually add the warmed milk, stirring continuously until you have a smooth, creamy sauce. Add the Gruyère, Dijon mustard, tarragon, and reserved bacon. Season well and allow to reduce until thick and glossy.

If using raw chicken, add it to the pan after the bacon, cook through, then remove before softening the vegetables and return it at this stage. If using pre-cooked chicken, stir it through the finished sauce now.

Loosen with a splash of the potato cooking water, then pour the filling into a large baking dish and leave to cool slightly.

Boil the potatoes in well-salted water until completely tender. Drain thoroughly, then mash with the butter until smooth and lump-free. Season well.

Spread the mash evenly over the filling, a piping bag gives the most even result. Using two forks, rough up the entire surface to create peaks. Bake at 200°C / fan 180°C / gas 6 until the top is deeply golden and the filling is bubbling at the edges.

Rest for five minutes before serving.