Mix together the rice vinegar, oil, soy sauce, agave, and garlic in a medium bowl. Add the portobello and toss to coat. Let the mushrooms marinate for 10 - 20 minutes, turning every now and then.
Heat a skillet over medium heat, add the mushrooms and the remaining marinade. Sauté for about 5 minutes, then flip the slices and sauté another 5 minutes so that both sides are caramelized and the mushrooms are tender.
Layer the sandwich with spinach and red onion, then top with the portobello slices. Drizzle with hoisin sauce. Best served hot.