Recipe

Make crust

1. Heat oven to 375 F. Finely zest lemon into a 9″ x 13″ or 3-qt. baking dish and set aside.
2. Whisk flours and ½ tsp of salt in large bowl. Cut butter into small pieces and sprinkle over flour. Use pastry blender or fingertips to combine, until mixture resembles coarse meal, with lumps smaller than the size of a pea.
3. Add yogurt, water, and 2 tsp of lemon juice (from half the zested lemon), and mix until large mass forms and there are no unmixed pockets of yogurt. Wrap with large piece of parchment paper and chill for 1 hour, or up to 2 days.

Prepare and bake leeks

1. Fill large bowl with cold water. Lop off darkest green parts of leeks and discard. Halve white through medium-green parts of leek lengthwise and fan open under water to remove any dirt and debris. Shake off excess water and cut into 2″ diagonal segments.
2. Add olive oil, 1 tsp of salt, many grinds of pepper and garlic to zest in baking dish, and mix. Arrange leek segments in dish, cut-side down, and cover tightly with lid or foil. Bake for 25 minutes, or until tip of knife can be easily inserted without meeting rubbery resistance. Transfer baking pan to cooling rack, carefully remove foil, and set aside.

Assemble Galette

1. Increase oven temperature to 400 F. Unwrap chilled dough and use parchment paper to line large baking sheet. On floured counter, roll dough out into about 14″-diam. circle. Gently transfer to parchment paper.
2. Spread Dijon mustard thinly on dough, leaving 2″ to 3″ border clear. Arrange brie slices over mustard, then season with salt and pepper. Use thin spatula to lift leek segments from baking dish, and arrange each, cut-side up, over brie. Scrape any juices left in pan over leeks. Season with more black pepper, then fold edges of dough in pleats over filling, leaving centre open. For darker, glossier crust, beat egg with 1 tsp of water and brush over crust.

Bake Galette

1. Bake for 30 minutes, until crust and leeks are a shade darker and brie is melted. Finish with squeeze of juice from remaining lemon half, cut galette into wedges and serve warm. This galette keeps in fridge for 3 days. Rewarm on baking sheet in 350 F oven for 10 to 15 minutes.