Potato-Gruyère Tart

Method

Place potatoes and 2 tablespoons of the salt in a large saucepan; add water to cover by 2cm. Bring to a boil over high; reduce heat to medium, and cook, undisturbed, until potatoes can be pierced using tip of a knife but are still firm, 2 to 3 minutes. Drain well; let stand until cool enough to handle, about 10 minutes. The potatoes need to be just tender but not completely soft when they are first boiled. If they get completely soft, they are a lot harder to cut into slices.

While the potatoes are cooking, make a start of the onions. Heat a non-stick pan to medium heat; add oil, and heat until shimmering. Stir in onion and remaining 1/4 teaspoon salt; cook, stirring occasionally, until translucent, about 3 minutes. Reduce heat to medium-low; cover and cook, stirring occasionally, until onion begins to soften and brown around edges, about 18 minutes. Make sure each time you remove the lid to stir, you let the droplets of condensation fall back into the onions in order to keep the heat and steam inside the pan. This helps the onions caramelize faster. Uncover and increase heat to medium-high; stir in water, scraping up any browned bits from bottom of saucepan. Cook 2 minutes, stirring often, until onion is browned and caramelized. Let stand until cool, about 5 minutes.

About 10 minutes before finishing the onions, preheat oven to 200°C with rack in lower third position. Line a rimmed baking sheet with parchment paper

Place puff pastry on a prepared baking sheet. Lightly score a border around perimeter of puff pastry using a paring knife, about 1 inch from edges. Prick pastry all over using a fork, leaving a 2-3cm border around edge.

Stir together cream, thyme, Dijon mustard, nutmeg, and 1 cup of the Gruyère in a medium bowl until combined. Spread cheese mixture in an even layer inside scored border of pastry. Arrange potato slices in overlapping layers over cheese mixture inside border; top evenly with caramelized onions. Top with remaining 1/2 cup Gruyère.

Bake in preheated oven until pastry is puffed and deep golden brown, about 30 minutes. Transfer baking sheet to a wire rack, and let cool 5 minutes. Garnish with black pepper and additional thyme. Slice into 16 pieces, less if you're already hungry; serve warm.