Step by step

1. Add the two types of flour and 3/4 teaspoon salt in the bowl of a stand mixer fitted with the Paddle Attachment. Mix on speed 1 until the dry ingredients are combined. Add the butter piece per piece while the mixer runs on speed 2, until it has broken down into pea-sized bits.

2. Pluck the leaves from 4 sprigs of thyme and 2 sprigs of rosemary and chop finely. Add to the flour mixture and put on speed 2. Add the water, one tablespoons at a time and mix until the dough just comes together. Wrap in cling film and refrigerate for at least 1 hour.

3. Cut the leeks in rings of about 1 cm thick. Heat the olive oil in a pan over medium heat. Cook the leeks until they are almost tender. Season with salt, pepper and nutmeg. Set aside and let cool off.

4. Melt the butter over medium heat and add the mushrooms and garlic to the pan. Season with the remaining salt, 1/8 teaspoon pepper and the leaves of 2 sprigs of thyme. Cook the mushrooms until they are slightly golden and not all the way cooked through yet. Set aside and let cool off.

5. Roll out the dough between 2 sheets of baking paper. Spread the mustard over the base, leaving about 2 cm around the edges. Grate the cheese over the mustard and arrange the leeks and mushrooms on top. Fold and pleat the edges of the dough border over the filling. Lightly brush the dough with a beaten egg.

6. Bake for 25 minutes in a 200°C preheated oven, rotating the galette after 15 minutes. Serve warm or at room temperature.