The trick to making this simple, stylish dish taste incredible is this: Don't rush. You want to really caramelize the onions to give the galette a silky texture and deep, rich, decadent flavor, and there are no shortcuts. But when you're done, you'll have a rustic, free-form version of a savory tart that's pretty enough to serve guests. Or make one for yourself on a quiet Sunday, and keep leftovers for weekday lunches. You can serve it with a slice with a simple side salad for a vegetarian-friendly lunch or dinner, or pair with your favorite eggs for a decadent brunch.
How to Make It
Preheat oven to 425ºF. Heat oil in a Dutch oven over medium. Add onions, thyme, and salt; cook, partially covered, stirring occasionally, until onions caramelize, about 45 minutes. Discard thyme stems. Stir in vinegar and pepper.
Unroll piecrust on a sheet of parchment paper, and use a rolling pin to roll out to a 13-inch circle. Pile onion mixture in middle of circle and spread evenly, leaving a 2-inch border; sprinkle cheese over onion filling. Fold edges of dough over filling.
Whisk together egg and water in a small bowl; brush over dough. Bake in preheated oven until crust is golden, about 20 minutes. Let cool 5 minutes before slicing.