1) Whisk the marinade ingredients together and pour it over your cleaned mushrooms. Pour the marinade in the gill side of the mushroom. Let them rest for at least an hour in the marinade.
2) Roast the mushrooms, gill side down, on a baking sheet at 400 degrees for about 15 minutes until they start to soften slightly.
3) For the stuffing, thaw the frozen spinach. Wrap the spinach in a paper towel and squeeze out as much moisture as possible. Then roughly chop up the spinach.
4) Heat oil in a large pan over medium-high heat. Once hot, add onions and button mushrooms and cook until the mushrooms are soft, a few minutes. Season with a pinch of salt.
5) Let your mushroom/onion mixture cool, then stir in your spinach and goat cheese.
6) Divide the filling evenly between the six mushrooms. There should be a huge mound of filling on each mushroom. Then sprinkle each mushroom with bread crumbs and some grated Parmesan cheese.
7) Bake these at 400 degrees for another 15-20 minutes until they are slightly browned on top.
8) Serve immediately!