Ingredients

Instructions

Make the pastry dough: Combine the flour and salt in a large bowl. Add the butter and work into the flour with your fingertips just like making biscuits until the mixture has the consistency of grated Parmesan cheese.

Mix the cold water in by the tablespoon, using your fingers until the dough holds together in a rough ball.

Flour a sheet of waxed paper or parchment paper. Dump the dough onto the paper. Flour the top of the dough and cover with another sheet of paper.

Place on a baking sheet and refrigerate 2 hours or overnight.

Let dough rest at room temperature for 10 minutes before rolling out.

Make the galette: Melt the butter in a large skillet over medium heat. Add the onions and cook until they begin to soften, about 5 minutes.

Add the garlic and keep cooking, stirring occasionally, until onions are deep golden brown, about 30 minutes more. Season with salt and pepper and stir in the mustard.

Heat the oven to 400°F.

Place the prepared dough on a sheet of parchment and roll it into a large rectangle that is about 14- by 10-inches. Lift the parchment and transfer it to a baking sheet.

Sprinkle the dough with ⅓ cup of the Parmesan. Then spoon the onions onto the dough and spread them into an even layer, leaving a 2-inch border bare around the edges. Fold the edges of the dough up and over the onions and brush them with the beaten egg.

Bake until the crust is just golden, about 10 to 15 minutes, then sprinkle with the remaining Parmesan. Return the pan to the oven and bake until deeply browned, about 15 to 20 minutes more.