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1
To make the tart shell, begin by combining the flour and salt in the bowl of a food processor. Add the cubed butter and pulse for about 5 seconds until well-combined. Pour in the ice water and pulse just until the dough forms a ball. Note: this can also be done by hand; use your finger tips to blend in the butter but be sure to work quickly so it doesn't melt.
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2
Form the dough into a ball and wrap in plastic. Freeze 1 hour or refrigerate overnight.
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3
Preheat the oven to 400u00b0F. Roll out the dough onto a floured work surface to create a circle 1/8-inch thick. Invert a 9-inch plate over the dough to measure the size of the tart and cut out a circle. Set the dough onto a silicone baking sheet or a piece of parchment paper on a baking sheet. Crimp the edges of the dough to make a decorative edge. Don't worry if the tart doesn't look perfect, it's supposed to be rustic.
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4
Set a piece of foil over the dough and fill with dried beans. Leave the edges exposed. In a small bowl whisk together the egg and water. Use a pastry brush to lightly coat the edges with egg wash.
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5
Bake the tart dough for 8 minutes. Remove the foil with the dried beans and use a fork to dock the bottom of the tart. Return the tart to the oven and bake for 10 minutes or until cooked through and lightly browned. Cool to room temperature.
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6
To make the filling, heat the olive oil in a large saute pan set over medium heat. Add the sliced onions and cook stirring often until soft and caramelized, about 15 to 18 minutes. Remove the onions from the pan.
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7
Melt the butter in the pan then add the plums. Cook stirring often until lightly roasted, about 5 minutes. Stir in the sugar and cook 2 minutes longer, then remove from the pan and cool to room temperature.
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8
To assemble the tart begin by spreading the mascarpone cheese in a thin layer over the shell. Top with a layer of caramelized onions then arrange the plum slices in a fan shape. Drizzle the honey and balsamic vinegar over the top and garnish with thinly sliced basil. Serve at room temperature.