Ensure your wood fired oven is fully saturated with a consistent flame.
Put the onion straight into the fire and the garlic bulb directly on the floor of the oven on the opposite side to your fire. After 20 minutes, remove both and allow to cool slightly before squeezing out the sweet inners and discarding the burnt outer of both.
Combine the roasted garlic with the mayo and horseradish and set to one side.
Next, preheat a pan in the oven and once hot, add the pancetta and diced chilli. Cook until beginning to brown then add half the butter and the mushrooms. Return the pan to the oven to cook for a further 5 minutes or so.
Preheat a second pan in the oven until smoking hot. Prepare the steak by making little nicks with a sharp knife in all the areas of fat, rub with a light olive oil and season with salt. Cook the steak to your liking, adding the remaining butter to the pan halfway through the cook time. Once out the pan, season with cracked black pepper and leave to rest.
Whilst your steak is resting, toast the ciabatta in the steak pan before spreading the horseradish aioli over both sides. Add the pancetta and mushrooms to the sandwich, chop up the onion and add that too.
Once the steak is rested, slice and layer over everything else in the sandwich. Top with sliced chilli and the other half of ciabatta and serve.