Preheat the oven to 350 ° F. Remove the stems from the portabellas and wipe them clean with a paper towel. Place the mushrooms on a plate and brush both sides with balsamic vinegar.
Cut the bell peppers into quarters and spray them with nonstick cooking spray. Place the pepper quarters on a cookie sheet and roast them in the oven for 30 minutes. Combine the vegetable broth and garlic in a sauté pan. Cook until the garlic is soft but not browned. Add the baby spinach and cook it until it is wilted. Season with salt.
Place the Portobello’s on a baking sheet, sprinkle them with the pepper and roast them in the oven for 10 minutes. While the mushrooms are cooking, place the remaining balsamic vinegar in a small saucepan or sauté pan. Reduce the liquid over medium heat until the mushrooms are done.
Layer the spinach and then a piece of bell pepper on top of each mushroom, and place them back in the oven for 5 minutes. Remove the mushroom napoleons from the oven. Place them on plates with a spatula, drizzle each one with the balsamic reduction, and sprinkle with the fresh herbs.