Prepare the Dough:

In the bowl of a food processor fitted with the steel blade, combine the flour, salt, and sugar. Pulse a few seconds to mix.

Add the diced butter and pulse 10 to 12 times until the butter is in small pea-sized bits.

With the motor running, pour the ice water down the feed tube and pulse just until the dough starts to come together.

Transfer the dough onto a floured board, knead quickly into a ball, wrap in plastic, and refrigerate for at least 1 hour.

Prepare the Tart:

Preheat Oven:

Preheat the oven to 400°F (200°C) and line a sheet pan with parchment paper.

Roll and Trim Dough:

Roll the dough between two sheets of lightly floured wax paper to a size slightly larger . Use a ruler and a small knife to trim the edges.

Place the dough on the prepared sheet pan and return it to the refrigerator while preparing the onions.

Add Cheese and Herbs:

Sprinkle the rolled pastry with the shredded cheese, covering right to the edges.

Evenly distribute the minced thyme and chives over the cheese.

Prepare and Arrange Onions:

Peel, halve, and very thinly slice the onions into half moons, keeping the slices intact.

Arrange the onion half moons on the pastry in diagonal lines, just barely overlapping.

Brush the onions lightly with cream.

Add Final Toppings and Bake:

Dot the tart with diced butter and sprinkle with kosher salt.

Bake for 40 minutes, or until the tart is golden and browned. If necessary, cover the edges with foil during baking to prevent excessive browning.

Let the tart cool slightly on the pan.

Cut into squares and serve warm or at room temperature.