For the pastry, whizz the flour and butter together in a food processor until it looks sandy. Add the thyme, mustard powder and egg. Pulse until the pastry comes together, adding 1-2 tbsp cold water to help it come together if necessary. Flatten, wrap in baking parchment and chill for 30 mins.

To make the filling, heat the butter and oil in a frying pan until the butter is foaming. Add the onions and a generous pinch of salt, then fry over a low-medium heat for 12-15 mins until golden and lightly caramelised. Leave to cool on a plate.

Cut the potatoes as thinly as possible (a mandoline will help here). Bring the cream, milk, garlic and thyme to the boil in a saucepan. Remove from the heat and add the potato slices. Mix well, so that each slice is coated in the cream. Season generously with salt and pepper, then leave to cool to room temperature.

Heat the oven to 200C/180C fan/gas 6 and put a baking sheet in the middle of the oven to heat up. Roll the pastry out in between two pieces of baking parchment to a 40cm round, about 5mm thick. Chill for 10 mins while you prepare the filling.

Strain the potatoes through a sieve, catching the excess cream in a bowl. Remove the top layer of baking parchment from the pastry, then add a layer of onions to the middle. Layer up the strained potatoes on top, leaving a 10cm border. Fold the border over and onto the potatoes, then add 3 tbsp of the leftover cream in the bowl to the potatoes. Use a little more of the cream to glaze the exposed pastry.

Using the parchment to help you, slide the galette onto the hot baking sheet. Bake for 40-45 mins until a knife slides easily into the centre, and the pastry is golden and crisp.

To make the cheese sauce, heat the butter in a saucepan until foaming. Stir in the flour and cook for 1 min. Gradually add the milk, a splash at a time, whisking constantly to ensure the sauce doesn’t go lumpy, until it’s all incorporated. Stir in the rest of the cream from the potatoes. Add the bay leaf and allow to simmer for 2-3 mins, stirring until you get a smooth white sauce. Take off the heat, then stir in the cheddar, nutmeg and mustard. Season with a little salt and pepper. You can warm this up to serve – add a splash more milk if you need to, to get a spoonable sauce.

Garnish the galette with more thyme, then cut into chunky wedges. Serve with the cheese sauce and buttered greens.