First make your slaw: add the vinegar, water, sugar, salt and mustard seeds in a pan and bring to a boil until the sugar dissolves. Let it cool then pour over the chopped vegetables and set aside in the fridge.
In a pan, heat a tablespoon of olive oil on a medium heat and fry up the onions until translucent.
Add the mushrooms and the paprika, onion powder, salt and pepper and stir for 5 minutes until the mushrooms begin to brown.
Turn up to a higher heat and add the pepper and minced garlic and stir continuously for 2 minutes.
Add the soy sauce and the cheese and stir together until the cheese melts into the mixture.
Toast the inside of the buns and slide the mixture into the buns.
Serve with the slaw on the side.