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• Preheat oven to 400°F. Place rack on top shelf.
• Slice onions into ¼-inch half-moons.
• In a large sauté pan over low heat, melt butter with 2 tbsp olive oil. Add onions, thyme sprig, and 1 tsp salt. Cover and cook for 15 minutes, stirring every 5 minutes, until translucent.
• Uncover the onions and raise the heat to medium. Stir occasionally until onions are golden and jammy (about 10–15 minutes more).
• Deglaze the pan with wine, scraping up browned bits. Reduce until liquid is absorbed. Remove the thyme sprig and adjust the seasoning.
• Meanwhile, roll out thawed puff pastry to a 9x14" rectangle. Place on a foil- and parchment-lined baking sheet. Prick with a fork, leaving a 1" edge untouched.
• Prebake pastry for 15 minutes. Rotate the tray and bake another 5–8 minutes until golden. Watch this carefully! 
• Assemble flatbread: Spread onion mixture over crust. Top with cheese, thyme, nutmeg (if using), and drizzle with remaining 1 tbsp olive oil.
• Bake again for 10 minutes, until cheese is melted and bubbling.
• Slice into 12 pieces and serve warm.

Tips and Tricks: 

Total onion cook time is 25–30 minutes. Be patient - caramelization takes time, but it is worth it!

The puff pastry can overbake if left too long in the second round; keep an eye on it.