Caramelized Onion and Horseradish Mashed Potatoes

Bahan-bahan (10 porsi)

• ¼ cup (½ stick) butter, (divided)
• 4 cups chopped onion
• 2 teaspoons brown sugar
• 1 tablespoon white balsamic vinegar ((optional))
• 2½ pounds peeled and cubed baking potatoes
• ½ cup whole milk
• ¼ cup Dijon mustard
• 1 tablespoon freshly squeezed lemon juice
• 2 tablespoons prepared horseradish
• 1 tablespoon light mayonnaise
• ½ teaspoon salt