Step 1

In a large skillet over low heat, melt butter. Add onions and stir to coat. Cook for 5 minutes, then add garlic. Cook, stirring occasionally, until onions are soft and golden brown, 30 to 35 more minutes. Add pepper and salt to taste. Stir in mustard.

Step 2

Preheat oven to 400°F. Meanwhile, with pie dough still between the 2 pieces of wax paper, roll it out to a 14- by 10-inch rectangle. Remove wax paper and place dough on a baking sheet lined with parchment.

Step 3

Sprinkle the dough with 1/3 cup Parmesan. Pile onions in the center of the dough; then spread them out, leaving a 2-inch perimeter. Fold the dough up and over the onions, pleating as necessary. Brush dough with egg.

Step 4

Bake on the bottom rack until the crust just begins to turn golden, 10 to 15 minutes; sprinkle the top with remaining Parmesan. Bake for 10 to 15 more minutes, then broil until crust is a deep brown, 3 to 5 minutes.