Directions:

Preheat oven to 425 degrees F. 

For the crust: Combine the flour, butter, Parmesan and a pinch of salt in the bowl of a food processor fitted with a pastry blade. Pulse a few times until the mixture resembles fine gravel. Add up to 8 tablespoons of ice water, a few tablespoons at a time, until the dough comes together and forms a ball. Remove from the food processor and form the dough into a disk; wrap it in the plastic wrap and refrigerate for at least 30 minutes or until ready to use.

For the tart: In a large saute pan over medium heat add 1 tablespoon olive oil. When the oil is hot, add the shallot rings, cippolini and pearl onions. Season with salt and pepper. Cook until the onions are softened but don’t have any color, about 5 minutes. Do not stir the pan or the shallot rings will break up, just shake it slightly every minute or so. Pour in the sherry and sprinkle the thyme over the top. Bring the mixture to a boil and continue to cook until the sherry has reduced almost completely, leaving only 3-4 tablespoons, about 10-15 minutes. Remove from heat and scatter the butter over the top and let it melt into the onions. 

Meanwhile on a lightly floured surface, roll the dough into a 15” round. Transfer the dough to a parchment lined sheet tray. Brush the dough with the remaining tablespoon of olive oil and season with salt. Top the dough with cheese, leaving at least a 1 ½ inch border around the edges. Then top the cheese with the shallot rings, cippolini and pearl onions. Scrape any sauce that remains in the pan over the onions. Fold the edge of the dough over on itself to seal the outside of the tart. Bake in the oven 30-35 minutes. Remove from oven, let cool slightly, and then serve.