1. Heat olive oil and butter in a large skillet over medium heat. Add the sliced onions and kosher salt, then cook for 10 minutes until softened.

2. Reduce the heat to medium-low and continue cooking the onions for 25 to 35 minutes, stirring often, until golden, sweet, and caramelized.

3. Stir in the garlic, thyme, black pepper, and balsamic vinegar. Cook for 1 to 2 minutes, then remove from the heat and let cool.

4. Line a baking sheet with parchment paper and roll the dough into a 16 by 10 inch rectangle on a lightly floured surface.

5. Mix the softened cream cheese with 1 1/4 cups Gruyère cheese, then fold in the cooled caramelized onions.

6. Spread the onion and cheese filling over the dough, leaving a small border around the edges.

7. Roll the dough from the long side into a log and pinch the seam to seal.

8. Shape the log into a wreath on the prepared baking sheet and pinch the ends together.

9. Cut slits around the outer edge of the wreath, then gently turn each section slightly to expose some of the filling.

10. Whisk the egg with water and brush it over the wreath.

11. Sprinkle with the remaining Gruyère, chopped rosemary, and flaky sea salt.

12. Bake at 375°F for 25 to 32 minutes, until puffed, golden brown, and cooked through.

13. Let the wreath rest for 8 to 10 minutes before serving warm.