Preheat the oven to 375º.

To caramelize the onions and fennel, heat a sauté pan over medium heat and add the olive oil and butter. Sauté until they are nicely caramelized, stirring occasionally, about 20 minutes. If browned bits of fond build up on the bottom of the pan deglaze with a splash of water.

Meanwhile, in a medium-size bowl, whisk together the heavy cream, egg yolks and thyme.

Once the onions and fennel are caramelized to a rich golden color, deglaze with the sherry and reduce until dry. Season to taste with salt and pepper.

Mix the onion/fennel mixture into the bowl with the eggs and cream.

Roll the disk of dough out on a floured surface into a 12-inch circle. Transfer to a parchment-lined sheet tray. Spread the onion/fennel mixture evenly over the bottom, leaving a 1-inch border of dough. Sprinkle the cheese on top.

Fold the edges of the dough over the edges of the filling to create a rustic-style crust.

Bake until golden brown and bubbly, about 40 minutes.