Melt the butter in a large skillet over medium heat. Add the onions and cook, stirring occasionally, until the onions begin to brown on all sides; about 10-12 min. Add the brown sugar and gently stir in the mustard to make a smooth coating over the onions; add salt, pepper, and 1 tbs parsley. Cook about 2min. longer. Place into a serving bowl and sprinkle with remaining 1 tbs parsley for garnish.