Ingredients
FOR THE PICKLED ONIONS
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7 tablespoons unsalted butter, divided
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1 small red onion (about 5 ounces)
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1/2 cup rice vinegar, white wine vinegar, or apple cider vinegar
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1/2 cup water
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1/2 teaspoon granulated sugar
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1/2 teaspoon fine salt
FOR THE TOASTS
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4 Wasa crispbreads
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4 teaspoons Dijon mustard
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1 (3.5 ounce) can sardines
