• 1/4 teaspoon salt
• 6 tablespoons unsalted butter, cut into small pieces and chilled (could probably use less butter and a healthier version)
• 5 tablespoons ice cold water
• 4 tablespoons unsalted butter(I used one tablespoon and it was plenty)
• 2 1/2 pounds sweet onions, thinly sliced
• 6 thyme sprigs (or chives or whatever you like with onions)
• 2 tablespoons crčme fraîche (hard to find so use heavy cream with dash of lemon juice)
• Salt and freshly ground pepper
• 1 egg beaten with 1 tablespoon of milk
Make the Dough:In a bowl, whisk the flour with the salt. Using your fingers, rub the butter into the flour until the mixture resembles coarse meal. Drizzle the water over the flour and stir gently just until incorporated; gently press to form a dough. Flatten the dough into a disk, wrap in plastic and refrigerate for 1 hour.
Meanwhile, prepare the filling:In a skillet, melt the butter. Add the onions and thyme and cook over moderately high heat, until softened, about 10 minutes. Reduce the heat to moderately low and cook, until the onions are golden, 20 minutes longer. Remove from the heat and discard the thyme. Stir in the crčme fraîche, (if using chives add here), and season with salt and pepper. Let cool.
Set a pizza stone on the bottom of the oven or position a rack on the lowest rung and preheat the oven to 375°. Line a large baking sheet with parchment paper. On a floured work surface, roll out the dough to a 12-inch round and transfer to the baking sheet. Spread the onions on the round, leaving a 1 1/2-inch border. Fold the edge of the dough up and over the filling and brush the edge with the egg wash.* I couldn't roll the dough so I spread it with my hands, also helps to use wax paper while working on the dough so it's easier to transfer).
Bake the tart on the stone or on the bottom shelf for about 40 minutes, until the dough is richly browned on the bottom. Transfer the tart to the top shelf and bake for about 5 minutes longer, until the top of the crust is browned. Transfer the tart to a rack and let cool slightly. Cut the warm tart into small wedges and serve.
*I think this may taste good with red grapes as a topping as well.