Preheat the oven to 350 degrees F. Wipe the mushrooms with a clean, damp cloth and trim stems. Slice the mushrooms into pieces no larger than 2 inches in size. Mix the olive oil with the garlic, herbs and seasonings in an ovenproof casserole dish large enough to fit all of the mushrooms. Add the mushrooms, and mix well, coating the mushrooms with the flavored oil. Bake for about 30 to 40 minutes, or until cooked through. Remove from the heat, and drizzle with the balsamic vinegar.