In a food processor, combine flour and kosher salt. Add cold butter and pulse until resembling coarse crumbs. Gradually add ice-cold water until the dough starts to clump. Shape into a disc, wrap in plastic, and refrigerate for at least 1 hour.

Rinse dandelion greens thoroughly and chop. Heat olive oil and butter in a skillet, add diced onion and sauté until tender. Stir in minced garlic and chopped dandelion greens until wilted. Set aside to cool.

In a bowl, whisk eggs and heavy cream. Season with salt, pepper, and lemon zest. Fold in sautéed greens, crumbled feta, and grated parmesan.

Preheat oven to 400°F (200°C) and grease a tart pan. Roll out chilled dough to fit the pan, trim excess, and dock the bottom with a fork.

Line crust with parchment and fill with weights. Pre-bake for about 15 minutes until golden. Remove weights and fill with green mixture, then pour cream mixture over the top.

Bake for 35 to 45 minutes until filling is set and top is golden brown. Cool for 10 minutes before slicing.