First make the dough, Sift together flour and salt in bowl and add to clean food processor with a dough attachment. Add small cubes of butter and blend together in food processor until it looks like small peas.
In another small bowl, whisk together the Greek yogurt, lemon juice and water and add this to flour mixture. Blend together and test by squeezing a small piece of dough with your hands. If it sticks, it's ready.
Using very floured hands, form dough into a disk, wrap in plastic wrap and allow to rest in the fridge for 30 minutes.
Next, make the vinaigrette. In a bowl, whisk together all the ingredients and taste for seasoning. Add sliced potatoes to the vinaigrette and toss gently to combine so all the potatoes are coated in the dressing. Set aside.
To make the leeks, in a large skillet add the butter and leeks and cook leeks on medium heat until just about wilted. Season with salt and pepper. The leeks should still have their green color and not be overcooked. Set aside.
Then make the bechamel. In a small pot, whisk together the butter and flour until there is no raw flour left. Add the mustard, paprika, salt and pepper and whisk to combine. Then slowly stir in the heated milk and continue stirring on low-medium heat until the milk thickens and mixture coats the back of a spoon, about 2-3 minutes. When ready, taste for seasoning and set aside.
When dough is ready, roll out on a lightly floured surface to about 12 inches in diameter or make 2 smaller ones (like I did this time).
Once dough is rolled out, transfer it to a non-stick baking sheet before filling. Spread mustard bechamel on dough leaving a 1 in border around, then layer with sauteed leeks and sliced potatoes.
Fold the border over, creating a pattern if you wish. Brush egg wash around the crust and sprinkle with sea salt.
Bake galette at 400 degrees F for 20 minutes until the crust is golden brown and potatoes cooked.