In a mixing bowl, combine flour and salt. Add cold butter and mix until the mixture resembles coarse crumbs. Stir in ice water until the dough comes together. Form into a disk, wrap in plastic, and refrigerate for 30 minutes.

In a skillet, heat olive oil over medium heat. Add sliced red onions and cook, stirring occasionally, for about 20-25 minutes until they are soft and caramelized. Stir in balsamic vinegar and season with salt and pepper.

Preheat the oven to 375°F (190°C). Roll out the chilled dough and fit it into a tart pan. Spread the caramelized onions evenly over the crust. In a bowl, whisk together eggs, heavy cream, thyme, salt, and pepper. Pour the mixture over the onions and sprinkle with goat cheese.

Bake in the preheated oven for 30-35 minutes or until the filling is set and the top is golden brown. Allow to cool slightly before slicing.