Rinse spinach and dry with paper towels or spin in the salad spinner.
Heat olive oil in Teflon pan. When it is heated, add sliced onion,garlic and fresh mushrooms. If you are using canned or jarred mushrooms, add them last with the balsamic vinegar. Sauté them until the onions are translucent (not brown).
Add spinach by the handfuls and let it wilt in the oil, stirring occasionally to wilt it evenly. When the spinach is melted, add the balsamic vinegar. Stir to coat the entire dish. Serve immediately.