In a food processor, combine wheat flour, sugar, and salt until blended. Add cold, diced butter and pulse until the mixture resembles coarse crumbs. Mix egg yolk and milk together, then drizzle into the processor, blending until just combined. Transfer dough to a pie pan, pressing it into bottom and up the sides. Chill for at least 1 hour.

Preheat your oven to 400°F (200°C). In a baking dish, toss chopped rhubarb with sugar, vanilla paste, and cardamom until well-coated. Cover the dish with foil and bake for 20 minutes. Remove the foil and bake for an additional 5-10 minutes, until tender but still holds shape. Set aside to cool.

Raise the oven temperature to 375°F (190°C). Line the chilled crust with parchment paper and fill it with pie weights or dried beans. Bake for 10-12 minutes until set, then remove the weights and parchment. Continue baking uncovered for an additional 3-5 minutes until lightly golden.

In a blender, combine sugar, yogurt, egg yolk, lemon zest, and lemon juice. Blend until smooth and slightly frothy.

Spread cooled rhubarb evenly over the baked crust. Pour lemon filling over the rhubarb, ensuring it covers all the fruit. Bake at 375°F (190°C) for 20-25 minutes, until edges are set with a slight jiggle in the center.

Allow the pie to cool completely on a wire rack for 2-3 hours. For optimal texture, refrigerate before serving. Slice and enjoy chilled, with whipped cream if desired.