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1
Preheat the oven to 375 degrees F.
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2
Heat an oven-proof 10-inch nonstick skillet over medium-high heat.
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3
Add the olive oil and onions and reduce the heat to medium.
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4
Cook, stirring often, until the onions are a dark golden-brown, about 15 minutes.
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5
Season with 1/2 teaspoon kosher salt.
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6
Stir in the balsamic and sprinkle the oregano over the top.
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7
Turn off the heat and spread the onions evenly over the bottom of the pan, using a rubber spatula.
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8
Place the ricotta in a small bowl.
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9
Season with 1/8 teaspoon each kosher salt and black pepper.
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10
Dollop the ricotta over the onions in 8 to 10 places.
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11
Using a fork, prick the puff pasty a few times evenly.
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12
Place the puff pastry over the top of the filling, pressing down gently to make sure the dough is touching the ricotta and onions.
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13
Bake until the pastry is puffed and golden brown, 35 to 40 minutes.
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14
Remove from the oven and cover with a plate slightly larger than the skillet.
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15
Invert the crostata onto the plate, replacing any onions that may have stuck to the pan.
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16
Set aside to cool slightly.
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17
Meanwhile, in a small bowl, dress the arugula with the lemon juice.
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18
Sprinkle the arugula over the cooled crostata and nestle the tomatoes into the onions.
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19
Slice into wedges or squares and serve.
