Preheat the oven to 400 degrees F and set the puff pastry aside to defrost.
Wash and clean the leek, making sure to remove any dirt, and slice thinly, using only the white and light green parts. Heat a pan over medium heat with a tablespoon or so of olive oil and add the leeks. Season lightly with salt and cook just until soft, 5-10 minutes.
While the leeks are sweating, peel the beets and slice thinly, about 1/8-inch thick.
Once the leeks are soft, remove them from the pan, and add the beets, adding more olive oil if needed. Season with salt and pepper and add the balsamic vinegar. Turn the heat down to medium-low and let the liquid reduce, 5-10 minutes.
While the beets are cooking, prepare the filling. Place the ricotta, egg yolk, gruyere, and lemon zest in a bowl and mix to combine. Season with salt and pepper.
On a well-floured surface, roll out the dough. Lightly score a border about 1/2-inch from the edge of the dough. Brush the dough all over with the egg white and place in the oven for 3 - 5 minutes or just until the egg white has set. (This will keep the dough from getting soggy)
Spread the ricotta mixture evenly over the dough, making sure to not go beyond the scored edge. Top it with the leeks, then the beets. Roast 20 minutes, remove from the oven and add the radishes and radicchio. Roast 5 to 10 minutes more until the beets are tender and the dough is golden around the edges.
Top with arugula or any fresh herb like parsley, chives or basil and serve warm or at room temperature. Enjoy!