Start by cutting off any particularly large or woody stems from the Portobello mushrooms.
Thinly mince or crush the garlic into a mixing bowl and add the rest of the marinade ingredients.
Using a teaspoon or pastry brush, cover the mushrooms with the garlicky oil marinade and brush thoroughly into all the cracks and gills. (They can be left in the fridge to marinate for up to 6 hours, if you would like to do this part ahead of time.)
Preheat a frying pan or the grill setting of an oven, and fry or grill the mushrooms on a medium to high heat for 8-10 minutes, until they have given up some of their water and become juicy and crispy.
Top with the plant-based cheese slices for the last few minutes of cooking time to melt the cheese.
Toast the burger buns and assemble the burgers! We suggest slices of tomatoes (lightly salted), gem lettuce leaves, cucumber pickles, onion rings and plenty of tomato sauce and plant-based mayonnaise.