- Heat the oven to 375° F (190° C). Finely grate the zest of your lemon into a nine-by-13-inch or three-quart baking dish, and set aside.
- Whisk the flour and half a teaspoon of salt in a large bowl. Cut the butter into small pieces, and sprinkle them over the flour. Use a pastry blender or your fingertips to squash the butter into the flour until the mixture resembles a coarse meal, with the biggest pieces of butter the size of tiny peas. Add the yogurt, water, and two teaspoons lemon juice (from half the zested lemon), and stir this into the butter-flour mixture until a large mass forms, with no unmixed pockets of yogurt.
- Wrap it with a large piece of parchment paper—we will use the paper again—and chill for one hour, or up to two days.
- Fill a large bowl with cold water. Lop off the darkest green parts of the leeks; we will use the white through medium-green parts for this galette. Halve the usable leek parts lengthwise, and fan them open underwater, letting any dirt and debris fall to the bottom of the bowl. Shake off the excess water, and cut them into two-inch diagonal segments.
- Add olive oil, one teaspoon salt, many grinds of pepper, and garlic to the zest in the baking dish, and mix.
- Arrange the leek segments, cut side down, in the dish, and cover with a lid or tightly with foil.
- Bake for 25 minutes, or until the tip of a knife can be easily inserted without meeting rubbery resistance.
- Transfer the baking pan to a cooling rack, carefully remove the foil, and set it aside to cool to lukewarm while the crust finishes chilling.
- Increase the oven temperature to 400° F (205° C).
- Unwrap the chilled dough, and flatten the parchment paper that wrapped it on a large baking sheet. On a floured counter, roll the dough out into a large roundish shape, about 14 inches across. Gently transfer it to the parchment paper.
- Spread Dijon mustard thinly on the dough, leaving a two-to-three-inch border clear.
- Arrange the brie slices over the mustard, then season the brie with salt and pepper.
- Use a thin spatula to lift the leek segments from the baking dish, and arrange each, cut side up, over the brie.
- Scrape any juices left in the pan over the leeks.
- Season with more black pepper, then fold the edges of the dough in pleats over the filling, leaving the center open.
- For a darker, glossier crust, beat an egg with one teaspoon of water, and brush it over the crust.
- Bake the galette: bake for 30 minutes, until the crust and leeks are a shade darker and the brie is melted.
- Finish with a squeeze of juice from the remaining lemon half, cut the galette into wedges, and serve it warm. Enjoy!
Make ahead tip: This galette keeps in the fridge for three days. Rewarm on a baking sheet in a 350°F (175°C) oven for 10 to 15 minutes.
Tip: Leeks’ sizes range tremendously, even from week to week in the same store or market stand. Plus, some have a larger dark-green portion, which we won’t use here, and some only have a little, so weight is little guidance to how much usable leek you’ll get from each one. instead, if they look small either in width(one-inch-thick stalks) or usability(i.e., the white/light-green portion looks very squat, only a small proportion of the stalk), just grab an extra bundle.