Buttered Balsamic Rosemary Mushrooms

Ingredients

• 1 large or 2 small shallots, peeled and finely diced
• 3 tablespoons unsalted butter, divided
• 1 pound portobello mushrooms, sliced 1/8-inch thick
• 1/2 teaspoon kosher salt
• 1/4 teaspoon freshly ground black pepper
• 1 1/2 teaspoons finely chopped fresh rosemary or thyme leaves
• 2 tablespoons balsamic vinegar
• 1 tablespoon packed light brown sugar
• 1 tablespoon tamari or soy sauce
• 3 cloves garlic, finely minced or grated