INGREDIENTS
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1 cup all-purpose flour
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3/4 cup yellow cornmeal
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3/4 teaspoon fine salt
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1 stick cold unsalted butter, cut into 1/2-inch pieces
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3/4 cup plus 3 tablespoons shredded manchego cheese
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2 tablespoons extra-virgin olive oil
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1 large onion, thinly sliced
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2 1/4 pounds mixed heirloom tomatoes
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Kosher salt
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3/4 cup shredded mozzarella cheese
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1/4 cup mayonnaise
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3 tablespoons breadcrumbs
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3 tablespoons chopped fresh chives
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3 tablespoons chopped fresh parsley
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1 teaspoon chopped fresh thyme
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Freshly ground pepper