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1/2 cup thinly sliced red onion
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1/4 cup red wine vinegar
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16 ounces uncooked gemelli pasta
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1/2 cup mayonnaise
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2 tablespoons extra-virgin olive oil
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1 tablespoons Italian seasoning
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2 teaspoons Dijon mustard
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1 teaspoon kosher salt, plus more for cooking water
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1/2 teaspoon crushed red pepper
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2 small garlic cloves, grated (1/2 teaspoon)
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1 pint cherry tomatoes, halved (about 2 cups)
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4 ounces drained fresh mozzarella cheese pearls (3/4 cup)
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4 ounces sharp provolone cheese, cut into 1/4- to 1/2-inch cubes (3/4 cup)
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4 ounces salami slices, torn into bite-size pieces (3/4 cup)
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4 ounces thin pepperoni slices, quartered (3/4 cup)
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2/3 cup drained pickled pepperoncini slices
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Croutons
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Basil leaves (optional)