Adjust the oven rack to the middle position and heat the oven to 375°F. Melt the butter in a 12-inch nonstick skillet over medium heat, then cook the onion until golden brown and softened, about 10 minutes, stirring often to ensure even browning.

Add the potatoes, thyme, salt, and pepper to the skillet. Increase the heat to medium-high and continue to cook until the potatoes are tender and lightly browned on the edges, 6 to 8 minutes, stirring occasionally.

Stir in the sun-dried tomatoes and remove the skillet from the heat.

Spread the potato mixture into the tart shell in an even layer. Cut the Camembert wheel in half horizontally through the center to create two thin discs, then cut each disc into four wedges. Arrange the cheese wedges, rind-side up, over the top of the filling.

Place the tart on a baking sheet and bake until the crust is deeply golden and the cheese is melted and bubbling, 25 to 35 minutes, rotating the sheet halfway through.

Let the tart cool on the sheet for 10 minutes to allow the filling to set. Carefully remove the outer ring of the pan, slide a thin metal spatula between the tart and the pan bottom to loosen it, and slide it onto a serving platter. Serve warm.