In a food processor, combine flour and kosher salt, pulsing until mixed. Add cold butter, blending until mixture resembles small peas. Gradually pour in ice-cold water while processing until a dough forms. Shape dough into a disk, wrap in plastic, and chill in the refrigerator for at least 1 hour.

While the dough chills, rinse dandelion greens thoroughly under cold water until clean, removing any grit or dirt. After washing, chop greens coarsely and set aside.

In a large skillet, heat olive oil and a tablespoon of butter over medium heat. Add diced onions and cook until tender and translucent, about 5 minutes. Then, add minced garlic, stirring until fragrant, approximately 1 minute. Toss in chopped dandelion greens and sauté until wilted, incorporating lemon zest before removing from heat to cool.

In a spacious mixing bowl, whisk together eggs, heavy cream, kosher salt, and freshly cracked pepper until well combined and slightly frothy.

Preheat your oven to 400°F (200°C). Roll out the chilled dough on a floured surface until about ¼ inch thick. Place rolled dough into tart pan, pressing gently into edges. Line dough with aluminum foil, fill with pie weights or dried beans, and bake for 15 minutes.

Once the crust is lightly baked, remove from the oven and take out weights and foil. Spread sautéed greens mixture evenly across crust, followed by crumbled feta cheese. Carefully pour egg mixture over the top.

Return filled tart to the oven and bake for 35-45 minutes, or until crust is golden brown and filling is set. Keep an eye on the tart in the last 10 minutes to prevent over-browning. Once done, remove from the oven and let cool slightly before slicing. Serve warm or at room temperature.