To make the burgers, mix the mince, onion and thyme together, adding salt and pepper to season. If you wish, fry a small piece of the mix and taste it to make sure that the seasoning is just right for you.
Divide the burger mix into four patties and place in the fridge for a couple of hours to firm up.
Finely slice the root vegetables and fry in vegetable oil until crisp. Drain, season with salt and set aside so that they remain crispy.
Heat a frying pan with a little oil and fry the burgers for 4 to 5 minutes on each side - longer if you like them well done.
Place the burgers on a baking tray, add a thick slice of blue cheese and brown under the grill
Serve the cheeseburgers in a toasted bun with the root vegetable crisps, salad and mustard.