Heat some oil in a pan. Sauté the onions for about 3-5 minutes until translucent. Add 2 tbsp water, cover the pan and simmer over low heat for further 1-2 minutes.
In another pan, fry the mushrooms in some oil for about 6-7 minutes until they are nicely browned. Then deglaze with soy sauce, tossing to combine, and season with salt and pepper to taste. Set aside.
While the mushrooms are cooking, fry the burger patties according to the package instructions (or make your own, read recipe notes*), and toast the burger buns.
Layer the toasted burger buns with the burger patties, vegan mozzarella, your favorite bbq sauce, roasted onions, and mushrooms.