Preheat your oven to 350°F (175°C).

Melt unsalted butter in a large skillet over medium-high heat. Add sliced onions and leeks, and 1 tsp salt. Sauté for 5 minutes.

Stir in 1 tbsp brown sugar with the onions and leeks. Cook for another 5 minutes until mixture turns golden brown.

Pour in ½ cup vegetable broth and cook for 5 more minutes. Transfer to a bowl and set aside.

In the skillet, combine the remaining ½ cup vegetable broth, ⅔ cup white wine, and ⅔ cup cream with 1 tsp salt. Bring to a gentle boil and stir in 1 tsp balsamic vinegar, then remove from heat.

Layer half of the thinly sliced potatoes in a greased baking dish. Sprinkle with thyme and black pepper. Add caramelized onions and leeks, then top with remaining potatoes, thyme, and black pepper.

Pour the creamy sauce over the layered potatoes.

Cover the baking dish with foil and bake for 30 minutes. Remove the foil and bake for another 30-40 minutes until cheese is melted and bubbly.

Let the gratin rest for 15 minutes before serving.