Melt the butter in a large fry pan over medium heat, add in the sliced onions and garlic. Slowly sauté, stirring occasionally allowing the onions to brown. Stirring is important to get the onions to brown evenly. Once the onions are golden brown add in the balsamic vinegar and allow them to glaze. Now add in the mushrooms and cook for another 2 minutes. Next add in the baby spinach and remove the pan from the heat. Gently fold in the spinach, season with salt, pepper and allow to cool. Line a 3-inch muffin tin with the puff pastry. Fill the puff with the onion mushroom filling and bake in a 400-degree pre heated oven. Tarts should take 6 to 8 minutes. Once cooked remove, top with crumbled goat cheese and serve.