Step 1: Make the Tart Crust

1. Prepare the Dough: In a mixing bowl, combine the flour and salt. Add the cold butter cubes, and use a pastry cutter or your fingertips to cut the butter into the flour until the mixture resembles coarse crumbs.
2. Add Ice Water: Gradually add the ice water, one tablespoon at a time, mixing gently after each addition, until the dough just comes together. Be careful not to overmix; the dough should still have small bits of butter for a flaky crust.
3. Chill the Dough: Shape the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes. This step helps relax the gluten and makes the dough easier to roll out.

Step 2: Caramelize the Onions

1. Heat Oil and Butter: In a large skillet, heat the olive oil and butter over medium heat until melted.
2. Add Onions and Seasoning: Add the sliced onions and a pinch of salt, stirring to coat the onions in the oil and butter. Cook over medium-low heat, stirring occasionally, for about 40-50 minutes, or until the onions are deep golden brown and caramelized. If desired, add sugar to help with caramelization.
3. Adjust Seasoning: Once the onions are caramelized, season with salt and pepper to taste. Set aside to cool slightly.

Step 3: Prepare the Filling

1. Combine Goat Cheese, Cream, and Eggs: In a mixing bowl, whisk together the goat cheese, heavy cream (or crème fraîche), and eggs until smooth and well combined.
2. Add Herbs: Stir in the chopped thyme or rosemary. This adds a subtle earthy flavor that complements the caramelized onions and goat cheese.

Step 4: Assemble the Tart

1. Roll Out the Dough: Preheat your oven to 375°F (190°C). On a lightly floured surface, roll out the chilled dough into a circle large enough to fit a 9-inch tart pan. Press the dough into the pan, trimming any excess.
2. Blind Bake the Crust: Line the crust with parchment paper and fill with pie weights or dried beans. Bake in the preheated oven for 10 minutes, then remove the weights and parchment paper and bake for another 5 minutes, or until the crust is lightly golden.
3. Add the Filling: Spread the caramelized onions evenly over the bottom of the baked crust. Pour the goat cheese mixture over the onions, smoothing it out with a spatula if necessary.

Step 5: Bake the Tart

1. Bake: Bake the tart in the preheated oven for 30-35 minutes, or until the filling is set and the top is lightly golden. The center should be just set but still have a slight jiggle.
2. Cool Slightly: Let the tart cool for at least 10 minutes before slicing. This allows the filling to set and makes it easier to slice.

Step 6: Garnish and Serve

1. Garnish: Garnish the tart with fresh thyme or rosemary sprigs for a pop of color and extra aroma.
2. Serve: Slice the tart and serve warm or at room temperature. For a more decadent experience, drizzle with balsamic reduction or honey just before serving. A side salad with mixed greens or arugula pairs beautifully with this tart.