Preheat the oven to 375°F (190°C).

In a large bowl, combine flour and salt. Add the butter and mix with a pastry blender until the mixture resembles coarse crumbs. Gradually add ice water, mixing with a fork until dough forms.

Roll out dough on a floured surface into a large circle. Place on a baking sheet.

In a large skillet, heat olive oil over medium heat. Add onions and cook until caramelized, about 15 minutes. Season with salt and pepper.

Spread caramelized onions over the rolled-out dough, leaving a 2-inch border. Top with grated Comté cheese.

Fold the edges of the dough over the filling, pleating as you go. Brush the crust with beaten egg.

Bake the tart for about 45 minutes, or until the crust is golden brown and the cheese is bubbly. Let cool slightly before serving.