INGREDIENTS
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1 tablespoon unsalted butter, plus more for the dish
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3 hamburger or hot dog buns, split
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1 tablespoon olive oil
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1 small yellow onion, thinly sliced
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3/4 pound sweet Italian sausage, casings removed
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6 large eggs
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1 cup milk
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kosher salt and pepper
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1/4 cup fresh flat-leaf parsley, chopped
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1 1/2 cups (6 ounces) grated Swiss cheese