- 15 minutes
- 1 hour 30 minutes (plus cooling time)
- Heat the butter in a large pot set over medium heat. Add the onions, salt, pepper and thyme and cook, stirring frequently, until the onions cook down by half and are golden brown and tender, 45-55 minutes. Add the balsamic vinegar and stir to combine. Remove from the heat and let cool.
- Preheat the oven to 400°F and line a baking sheet with the parchment paper.
- Lay the pie dough on top of the parchment and spread a thin layer of Dijon mustard on the pie dough, leaving a 1-inch border.
- Sprinkle 1 cup of cheese atop the mustard, then spoon on the cooked and cooled onions. Top with the remaining cheese and gently fold the outer edges of the dough over the filling to create a freeform crust.
- Brush the dough with the beaten egg and bake for 35 minutes or until the dough is golden brown and set on the bottom. Let cool for 5-10 minutes before serving.
Recipe Tip: Pair this tart with a simple green salad for lunch or a light dinner.