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