2 sweet onions (such as Vidalia or Walla Walla), sliced into 14-inch-thick slices |
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kosher salt and freshly ground black pepper |
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1/4 cup aged sherry or balsamic vinegar |
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1/3 cup extra-virgin olive oil |
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2 teaspoons finely chopped fresh thyme leaves |
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2 tablespoons chopped fresh flat-leaf parsley leaves |
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4 (8-ounce) tuna steaks, cut into 1/4-inch-thick |
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