Reduce the heat to medium and add 1 piece of the unsalted butter to the skillet. Once melted, add the onion mixture and 2 teaspoons of the kosher salt. Cook, stirring occasionally, until the vegetables are tender, about 10 minutes. Add 1/4 cup dry white wine if using and cook until the liquid is evaporated, about 3 minutes. Transfer to the bowl with the sausage.