Start by heating the oil in a large frying pan and fry the finely chopped mushrooms on medium high heat for around 5 minutes, until they are completely cooked.
Add the crushed garlic to the pan and fry for a further minute. Then leave the mushroom mix to cool down.
Once cool, in a medium bowl, add the minced beef, mushrooms, ketchup, mustard, sea salt and pepper.
Mix really well until all Ingredients are incorporated and mixed together thoroughly.
Divide the mix into 4 and shape into burger patties. Then place the patties in the fridge to rest for 30 minutes. You can also create the burgers in advance the day before and keep them covered in the fridge.
Heat up the BBQ or a large frying pan with a little oil and fry the burgers for 2-3 minutes on each side until golden and caramelised on the outside (keep on for longer if you prefer them well done).
For the final minute, top with a slice of burger cheese to melt.
Once complete, set aside and fry the buns to lightly toast.
Build the burger by slathering the bottom of the bun generously with sauce, before loading with the patty, lettuce, onion, sliced tomato and pickles.