Heat olive oil in a large skillet over medium-high heat. Sauté onion and garlic in oil until tender. Add mushrooms and fry until they begin to shrink, about 3 to 4 minutes. Toss in spinach and sauté, stirring constantly for a few minutes, until spinach is wilted.
Add vinegar, stirring constantly until it is absorbed, then stir in white wine. Reduce heat to low and simmer until wine is almost completely absorbed.
Season with salt and pepper to taste and sprinkle with fresh parsley. Serve hot.