In a large skillet over medium-low heat, melt 2 tablespoons of unsalted butter with 1 tablespoon of olive oil. Add 6 medium sliced yellow onions, cover, and sauté until tender, about 18-20 minutes. Uncover, increase heat to medium, stir in 1/4 teaspoon sugar and a pinch of kosher salt, and cook until golden brown and caramelized for 20-30 minutes. Let cool, then mix in 1 tablespoon of crème fraîche and 1 1/4 cups each of grated Gruyère and Parmesan cheese.

In a mixing bowl, combine 2 cups of all-purpose flour, 1 teaspoon of kosher salt, and 1 packet of yeast. Slowly add 3/4 cup of warm water and 1 tablespoon of honey, along with 1 tablespoon of olive oil, mixing until combined. Knead the dough on a floured surface for about 5 minutes until smooth. Let it rest for 15 minutes.

Preheat your oven to 450°F (232°C). Roll out the rested dough on a floured surface into a rectangle that fits your greased baking sheet. Transfer the dough onto the sheet, spread the cooled onion mixture evenly over the top, and sprinkle the remaining cheese over the onion layer. Bake for 10-15 minutes until golden brown.

Once baked, remove from the oven and allow it to cool for a few minutes. Garnish with fresh thyme leaves to enhance the aromatic experience. Slice into pieces and serve warm.