In a large skillet, melt 1/2 tablespoon of the butter over medium heat. Add the 14.4 ounces frozen pearl onions, stirring and cooking for 15 minutes.
Stir the 1 tablespoon light brown sugar into the onions until fully coated. Stir in the 1 teaspoon Dijon mustard and 3 tablespoons balsamic vinegar. Cover and reduce the heat. Cook for another 15-20 minutes, stirring occasionally. If the onions get too dry, add 1 tablespoon of vegetable broth. You may not need the whole amount of broth.
Remove from the heat and stir in the remaining 1/2 tablespoon of butter, 1/4 teaspoon coarse Kosher salt and 1/8 teaspoon ground black pepper. Garnish with chopped fresh parsley. Serve hot.
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