Wash and dry all vegetables. In a flat-bottomed frying pan combine the onions, garlic, and 1 tablespoon of olive oil. Cook on medium heat for 10 minutes. Add the asparagus, balsamic vinegar, brown sugar, the last tablespoon of olive oil, and the salt and pepper. Mix the ingredients so the liquid is consistent around the pan. Keep on medium heat and cook for 10 minutes, stirring every few minutes to keep the asparagus coated in the liquid. If the liquid starts to reduce too quickly before you have added the mushrooms, add a little water. Lastly, add the mushrooms and cook for 5 more minutes or until the mushrooms have softened and the liquid has reduced down to a sticky coating over all the veggies.