Prepare the Pastry:

Combine flour and salt in a large bowl. Add the butter and use a pastry blender or fingers to mix until the mixture resembles coarse crumbs.
Gradually add ice water, stirring until the dough comes together. Form into a disk, wrap in plastic, and chill for at least 1 hour.

Pre-bake the Crust:

Preheat the oven to 375°F (190°C).
Roll out the dough to fit a 9-inch (23 cm) tart pan. Press the dough into the pan, trim the excess, and prick the bottom with a fork.
Line with parchment paper and fill with pie weights or dried beans. Bake for 20 minutes, remove weights and paper, and bake for another 10 minutes until lightly golden.

Prepare the Filling:

In a large skillet, heat butter and oil over medium heat. Add onions, sugar, and salt. Cook, stirring occasionally, until onions are caramelized, about 30-40 minutes. Stir in thyme.

Assemble the Tart:

Spread the caramelized onions evenly over the pre-baked crust.
In a bowl, whisk together eggs, cream, salt, and pepper. Pour this mixture over the onions. Sprinkle with cheese if using.

Bake:

Bake at 375°F (190°C) for 25-30 minutes, or until the custard is set and the top is golden brown.

Serve:

Let the tart cool slightly before slicing. Serve warm or at room temperature.