Preheat oven to 180 degrees Celsius. Line baking tray with a layer of tin foil and place de-stemmed mushrooms gill side up on the tray.
To marinade mushrooms, combine finely diced Onion, minced Garlic, Tamari, Olive Oil, Agave Nectar, Balsamic Vinegar and Black Pepper in a bowl. Whisk well to combine and then drizzle evenly into the cavity of each Portobello Mushroom. Leave to marinade for 30 minutes.
Meanwhile, make Sweet Fried Onions.
In a frying pan, heat up Olive Oil on low heat. Add diced Red Onion, Salt and Pepper to the pan. Cook whilst stirring until Onions turn translucent.
Once translucent, add sugar to the pan and stir well. Continue cooking onions on low/medium heat for 15-20 minutes or until lightly browned and sweet. Taste and check for seasoning, then set aside to keep warm.
Pop tray of Mushrooms into oven and bake for 20 minutes. After 20 minutes, remove tray from oven and drain excess liquid. Place Vegan Cheese Slices on the mushrooms and return to the oven. Bake until cheese has melted.
Serve Portobello Burgers hot, sandwiched in toasted buns and filled with sliced Tomato, Avocado and a generous serving of Sweet Fried Onions.