On a well floured surface, roll pie dough out into a circle about 12 inches in diameter. Transfer to a baking sheet lined with parchment. Spread Formag Blanc over the surface of the dough, leaving a 2 inch border all around. Evenly distribute onions over the Formag Blanc, then sprinkle with half of the gruyere. Fold up the ends of the crust up over the cheese and onions towards the center. Sprinkle entire galette, including the folded crust, with the remaining gruyere.
Bake in the preheated oven for about 30-35 minutes, until crust is cooked and cheese is browned.
Notes
For the freezer: caramelized onions, grated cheese, and dough can all be frozen in separate freezer bags. Let defrost, then proceed with assembly directions.
Recipe by Wright Family Table at https://www.wrightfamilytable.com/french-onion-galette/