INGREDIENTS
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2 1/4 teaspoons active dry yeast (a 1/4-ounce package)
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1/2 cup warm water (105-115°F)
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1 1/2 to 1 3/4 cups all-purpose flour
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1 large egg
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1/3 cup plus 1 tablespoon extra-virgin olive oil, divided
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2 1/2 teaspoons salt, divided
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2 teaspoons fennel seeds
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3 pound yellow onions, halved and thinly sliced
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1 tablespoon Dijon mustard
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1/2 cup grated Parmigiano-Reggiano