Heat 3 tablespoons oil in a large frying pan over a medium heat, then saute the onions for 4 minutes, until they start to soften. Add the garlic, mushrooms and thyme and cook for a further 3 minutes or until most of the mushroom juice has evaporated. Stir in the drained beans and soy sauce and cook through for around 2 minutes, stirring occasionally. Turn off the heat.
Roughly mash the mixture with a potato masher, whilst keeping some texture; be careful not to over mash. Stir in the flour and nutritional yeast flakes, if using. Divide the mixture into 4 portions. Then, using your hands, mold each portion into a burger shape about 1/2 inch thick and 3 inches wide or wide enough to fit your bun.
Put the burgers on a parchment-lined sheet pan. Allow to chill in the fridge for 1/2 hour, to firm up before cooking.
To fry the burgers: Heat the remaining 3 tablespoons oil in a medium-large frying pan over a medium heat (you want to hear a nice sizzle) and cook the burgers for about 2 minutes on each side or until cooked through and crisp brown on both sides.
To assemble the burgers: Spread 1 teaspoon of 1000 Island Dressing on the base of each bun, then lay a lettuce leaf on top. Add a burger, spread a thin layer of mustard onto each burger, add a slice or 2 of tomato, and then 3 thin slices of pickle. Add a few arugula leaves, then spread a little mustard on the underside of each burger top to complete. Serve with potato chips, if using.
1000 Island Dressing:
Mix all the ingredients together in a small serving bowl, whisking with a fork.
This dressing is delicious spooned over veggie burgers or served as a dip with crudites. I also love it drizzled over a festive vegetarian nut roast the next day in a sandwich. Any unused dressing can be stored in an airtight container in the fridge for up to 4 days.