In a medium bowl, combine olive oil, vinegar, soy sauce, rosemary, thyme, garlic and salt and stir well to combine. Transfer to a 1-gallon sealable plastic bag. Add red onion and mushrooms and seal bag, laying flat to ensure mushrooms and onions are sitting in the marinade. After 5 minutes, flip bag over so that the other side of the mushrooms absorbs marinade. Let sit another 5 minutes.
To serve, transfer each mushroom to a toasted bun. Top with cooked onion, spinach and roasted red peppers. Drizzle with additional thickened balsamic marinade.
Recipe by Prepgreen at https://www.prepgreen.com/portobello-mushroom-sandwich/