• Preheated oven to 375°F (190°C).

Caramelize the Onions

• In a large pan, sauté the onions over medium heat until golden. They should brown gradually over around 30 minutes. If needed, add a little more butter to help avoid burning.
• As the onion is starting to brown, add the sugar, balsamic vinegar, and the salt. Continue to cook a few more minutes until the onions are deeply caramelized and the liquid has evaporated. Set aside.

Make the Pastry

• In a large bowl, mix the flour and salt.
• Cut in the butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
• Add the milk and mix until the dough comes together.
• Roll out the dough and line the bottom of a Pyrex dish or tart pan.
• Beat the egg and brush it over the pastry.
• Bake in a preheated oven at 375°F (190°C) for 10 minutes, until partially cooked.

Assemble the Tart

• Evenly scatter the caramelized onions over the partially baked pastry.
• Place goat cheese rounds on top of the onions.
• Sprinkle with chopped rosemary.

Prepare the Custard

• In a bowl, whisk together the egg, heavy cream, nutmeg, salt, and white pepper.
• Pour the mixture over the onions and cheese in the tart.

Bake

• Bake in the oven at 375°F (190°C) for 20 minutes, or until the filling is set and the top is lightly golden.

Serve

• Allow the tart to cool slightly before slicing and serving. Enjoy!