paper towel.</li>
<li>While the onion cooks, make a horseradish sauce. In a bowl, mix the grated horseradish with the buttermilk and a pinch of salt. Stir to combine and refrigerate until ready to use.</li>
<li>Before you begin cooking the burgers, get the buns toasting. Heat a cast-iron skillet or similar surface over medium-low heat. Slice the buns in half horizontally. Smear the remaining 2 tablespoons of butter on the buns and place, butter side down, on the hot surface, working in batches if necessary.