Preheat broiler. Heat butter and oil in a non-stick 20 cm (8-inch) ovenproof skillet over medium heat. Add onions and cook for about 2 minutes or until golden. Stir in mushrooms and balsamic vinegar; cook for about 3 minutes. Lightly beat eggs with parsley and pepper. Pour into pan and lower heat to medium-low, stirring quickly just enough to mix in mushroom mixture. As mixture sets at the edges, with spatula, gently lift cooked portion to allow uncooked egg to flow underneath. Cook until the bottom is set and the top is almost set, about 4 minutes. Place under broiler for 2 to 3 minutes or until eggs are set. Cut into wedges to serve.