Grate beetroot with a fine grater then add the grated cheddar, broken up blue cheese and the mayo. Mix thoroughly, mashing the blue cheese into the mixture until it's well combined and you have a weird pink paste!
I like to use a cast iron pan - heat well, butter your bread slices on one side then spread with a little mayo, then add that side down to the hot pan. You can use a burger press to ensure good contact with the pan for an even golden, toasty base.
When toasted to your preference, take off the heat, flip over and distribute the beety, cheesy mix between the slices, spreading to the edges.
Have your grill preheating, and pop the whole pan under, until the mix is bubbling, golden and blistered.
Remove to plates, cut in half and top with some of the apple and broken walnut pieces.
