In a small skillet over medium heat, add the olive oil. Once heated, add the onions and cook over medium-low heat until soft and caramelized, stirring occasionally as needed. This will take about 15 to 20 minutes. Once they’re caramelized, add the mushrooms and cook for a further 5 to 7 minutes, until the liquid is released. Add the vegan Worcestershire sauce and cook for one minute more. Remove, season with salt and pepper and set aside.