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