Halve the burger patties and roll them into a rough ball. Season well with salt and pepper
Preheat a large non-stick frying pan over high heat until smoking hot
While the pan is heating, spread both sides of the bun with vegan mayo. Toast the cut side of each bun in the pan as it’s heating up-it shouldn’t take more than 30 seconds to get a nice golden colour. Set aside
When the pan is hot, add the burger balls, well spaced out from one another. Using two flat spatulas-one to cover the burger ball and the other on top to help push, flatten the patties until they are very thin. It can help to use a piece of non-stick baking parchment over the top to prevent your spatula from sticking
Fry for 2 minutes until a deeply caramelised crust has formed on the base. Squeeze a bit of mustard on the uncooked side of the burger bun, then flip. Turn down the heat, and lay a cheese slice over each burger patty. Splash a tiny bit of water into the pan, and cover it with a lid for 1-2minutes to help the cheese to melt faster and keep the moisture in your burgers
Mix together all the ingredients for the burger sauce and set aside
Assemble your burgers with a tablespoon of burger sauce on the base bun, your toppings of choice, two smash patties and the bun lid