In a mixing bowl, combine flour and salt. Add cold butter and mix until the mixture resembles coarse crumbs.

Gradually add ice water, mixing until the dough comes together. Form into a disc, wrap in plastic, and refrigerate for 30 minutes.

In a skillet, heat olive oil over medium heat. Add sliced red onions, sugar, and salt.

Cook, stirring occasionally, for about 20-25 minutes until onions are caramelized and soft.

Preheat the oven to 375°F (190°C). Roll out the chilled tart dough and fit it into a tart pan.

Spread the caramelized onions evenly over the crust. In a bowl, whisk together eggs, cream, and thyme. Pour over the onions.

Sprinkle crumbled goat cheese on top. Bake for 30 minutes or until the filling is set and the crust is golden.

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