Heat 1 tbsp (15 mL) oil in large skillet set over medium heat. Add mushrooms, sprinkle with 1/4 tsp (1 mL) salt and cook 8 to 10 min. until golden brown. Transfer to plate.
In same skillet, heat remaining 1 tbsp (15 mL) oil. Add onions and remaining 1/4 tsp (1mL) salt. Cook about 15 min., stirring often, until tender and golden. Add mushrooms back into skillet. Stir in vinegar and maple syrup. Cook 3 min., stirring often, until glaze is thickened and coats mushrooms and onions. Stir in parsley.
Serve as a topper for grilled hot dogs, sausages, burgers, veggies or pasta.