Place an oven rack in the center of the oven. Preheat the oven to 400 degrees F.
Butter and flour the bottom and sides of a 9-inch tart pan with a removable bottom. Unroll the pie crust and gently press into the bottom and sides of the pan. Trim any excess from the top.
With the tines of a fork, prick the bottom of the pastry. Using a pastry brush, brush the crust with the beaten egg white. Put the pan on a baking sheet and bake for 10 minutes, until the egg white has set. Let cool 10 minutes.
Meanwhile, in a medium skillet, heat the oil over medium-high heat. Add the pancetta and cook, stirring frequently, for 6 to 8 minutes, until browned and crispy. Drain on a paper towel-lined plate.
In a medium bowl, lightly beat the eggs. Whisk in the mascarpone, gruyere, scallions, salt, pepper and the pancetta. Pour into the cooled crust and bake for about 18 minutes, until the mixture has set and the top is golden.
Cool the tart for 10 to 15 minutes and then remove from the pan. Cut the tart into wedges and serve warm or at room temperature.