Place 200 g all-purpose flour, 60 g unsalted butter, and a pinch of salt in the stand mixer bowl.

Mix at medium speed until the butter and flour form a sandy texture.

Add 60 ml water to the flour mixture.

Mix just until the dough comes together into a ball.

Roll the dough into a circle slightly larger than the 25 cm tart pan.

Cover the dough and chill it in the refrigerator while preparing the onions.

Peel and cut most of the 10 red onions into quarters, reserving some onions for thin slicing.

Heat 1 tbsp olive oil in the frying pan over medium heat.

Arrange the onion quarters cut-side down in the hot pan.

Cook the onion quarters over medium heat for 10 minutes until one side is browned.

Add 2 tbsp maple syrup to the browned onion quarters.

Add 2 fresh thyme sprigs and a pinch of salt before the syrup becomes too dark.

Deglaze the pan with about 2 tbsp sherry vinegar.

Simmer the onion quarters over low heat until the liquid has evaporated.

Transfer the caramelized onion quarters from the pan to a bowl.

Thinly slice the reserved red onions.

Melt 10 g salted butter in the same frying pan.

Sauté the sliced red onions with 1 tsp sumac and a pinch of salt until softened.

Deglaze the sliced onions with the remaining 1 tbsp sherry vinegar.

Line the 25 cm tart pan with parchment paper.

Arrange the caramelized onion quarters in a tight spiral with their browned sides facing down.

Spread the sautéed sliced onions over the onion quarters.

Cover the onions with the chilled dough round.

Tuck the dough edges firmly inside the tart pan around the onion filling.

Bake the tart at 170°C / 340°F for 40 minutes until the crust is golden.

Let the baked tarte Tatin rest for 5 minutes before inverting it onto a serving plate.

Place 200 ml heavy cream, 1 tbsp mascarpone, a pinch of salt, and 1 tsp sumac in a cold mixer bowl.

Whip the cream mixture until it forms firm, compact peaks.

Serve the warm red onion tarte Tatin with the sumac whipped cream.

Garnish each serving with fresh thyme and fleur de sel.