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1
Preheat the oven to 375.
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2
Line a rimmed baking sheet with parchment paper.
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3
Spread the herbs and garlic on the paper and arrange the tomatoes on top in a single layer.
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4
Drizzle with 3 tablespoons of olive oil and season with salt and pepper.
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5
Roast for 1 1/2 hours, until the tomatoes are soft; discard the herbs.
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6
Increase the oven temperature to 450.
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7
Meanwhile, in a large skillet, melt the butter.
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8
Stir in the onions and sugar.
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9
Cover and cook over moderately low heat, stirring, until the onions are golden in spots, 10 minutes.
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10
Uncover and cook over moderate heat, stirring, until the onions are deep brown, 7 minutes longer.
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11
Season with salt and pepper and keep warm.
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12
Light a grill.
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13
On a floured work surface, using a lightly floured rolling pin, roll out each piece of dough into an 8-inch round.
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14
Grill each round until bubbles appear on the surface, 2 minutes, then flip and cook until golden and slightly puffed, 2 minutes longer.
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15
Transfer the flatbreads to 2 baking sheets.
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16
Spread the flatbreads with the nduja.
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17
Top with the tomatoes, onions and cheese.
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18
Bake for 7 minutes, until the cheese is melted.
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19
Top with the arugula and a sprinkling of olive oil, vinegar and fleur de sel.
