In a food processor, combine flour and kosher salt for the tart dough. Cut in cold butter until the mixture resembles coarse crumbs. Gradually drizzle in ice-cold water, pulsing until the dough just clumps together. Form the dough into a disc, wrap it in plastic wrap, and chill in the refrigerator for at least 1 hour.

Rinse the dandelion greens thoroughly under cold running water to remove any grit. Roughly chop the greens and set aside on a clean kitchen towel to dry.

Heat olive oil and butter together in a skillet over medium heat. Add the diced onion, cooking until softened and translucent, about 5-7 minutes. Stir in the minced garlic for an additional minute until fragrant. Gradually add the chopped dandelion greens, cooking in batches until wilted and tender. Mix in the lemon zest, then allow the mixture to cool.

In a mixing bowl, whisk together eggs, heavy cream, a pinch of salt, and freshly cracked black pepper. Combine until smooth and well blended.

Preheat your oven to 400°F (200°C) and prepare a 10-inch tart pan by greasing it with butter or cooking spray.

On a lightly floured surface, roll out the chilled tart dough to a circle about 1 inch larger than your tart pan. Transfer the dough into the greased pan, pressing it into the corners and trimming the excess from the edges. Dock the bottom with a fork.

Line the tart shell with parchment paper and fill it with pie weights or dried beans. Bake in the preheated oven for 15 minutes, then remove the weights and parchment. Return the crust to the oven for another 5 minutes.

Spread the wilted dandelion greens mixture evenly across the baked tart shell. Sprinkle the crumbled feta cheese over the greens, then pour the egg and cream mixture on top. Add shredded parmesan for flavor.

Place the assembled tart back in the oven and bake for 35-45 minutes. The tart is done when the filling is set and the top is bubbling and lightly golden. Allow it to cool for 5-10 minutes before serving warm.