Add flour, salt, and cubed butter to a food processor. Pulse until the butter is the size of small peas.

In a small bowl, whisk together the Greek yogurt, lemon juice, and ice water. Add the mixture to the food processor and pulse until the dough forms a ball.

Using floured hands, shape the dough into a disk, wrap it in plastic wrap, and chill in the refrigerator for 30 minutes.

Melt butter in a skillet over medium heat. Add the chopped leeks and sauté until wilted, about 5-8 minutes. Season with salt and pepper, then set aside to cool.

To make the béchamel, melt butter in a small pot over medium heat. Whisk in the flour and cook for 1-2 minutes until no raw flour remains.

Stir in Dijon mustard, paprika, salt, and pepper. Gradually add the warmed milk, whisking constantly until the sauce thickens and coats the back of a spoon, about 2-3 minutes. Taste and adjust seasoning if needed, then set aside.

Preheat the oven to 400°F (200°C).

Roll out the chilled dough on a lightly floured surface into a 10-12 inch circle, or divide it into two smaller tarts. Transfer the dough to a parchment-lined baking sheet.

Spread the béchamel over the dough, leaving a 1-inch border. Layer the sautéed leeks and thinly sliced potatoes on top.

Bake the tart for 25-30 minutes, or until the crust is golden brown and the potatoes are tender.

Garnish with fresh thyme leaves and serve warm.