1
For the dough, quickly combine the flour and butter in a food processor until the mixture becomes sandy. Add thyme, mustard powder and the egg. Pulse until the dough comes together, adding 1‑2 tbsp cold water if needed to help it bind. Shape into a disc, wrap in parchment for baking and chill for 30 minutes.
2
For the filling, heat butter and olive oil in a pan until the butter begins to foam. Add the onion and a generous pinch of salt, then sauté over medium‑low heat for 12‑15 minutes until golden and lightly caramelised. Let cool on a plate.
3
Slice the potatoes as thinly as possible (a mandoline helps). In a saucepan bring the cream, milk, garlic and thyme to a boil. Remove from heat and add the potato slices. Stir well so each slice is coated with the cream. Generously season with salt and pepper, then let cool to room temperature.
4
Preheat the oven to 200 °C (180 °C with convection) / gas mark 6 and place a tray in the middle of the oven to heat up. Roll out the dough between two sheets of parchment to a round 40 cm diameter, about 5 mm thick. Chill for 10 minutes while you prepare the filling.
5
Pass the potatoes through a sieve, collecting the excess cream in a bowl. Remove the top layer of parchment from the dough, then spread a layer of onion in the centre. On top, add the sieved potatoes, leaving a 10 cm border. Fold the edge over the potatoes, then add 3 tbsp of the remaining cream from the bowl onto the potatoes. Use a little more cream to glaze the exposed dough.
6
Using the parchment, carefully transfer the galette onto a hot tray. Bake for 40‑45 minutes until a knife slides through the centre easily and the dough is golden and crisp.
7
For the cheese sauce, heat butter in a saucepan until it foams. Add the flour and cook for 1 minute. Gradually whisk in the milk, drop by drop, constantly stirring to avoid lumps until fully combined. Add the remaining potato cream. Add a bay leaf and simmer for 2‑3 minutes, stirring constantly, until you have a smooth white sauce. Remove from heat, then stir in cheddar, nutmeg and Dijon mustard. Season with salt and pepper. If needed, warm before serving – add a little more milk to achieve a sauce that coats the spoon easily.
8
Garnish the galette with extra thyme, then cut into large wedges. Serve with the cheese sauce and butter‑sautéed greens.

Source: bbcgoodfood.com

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