Trim the stalk and base off each mushroom, giving you a beautiful cross-section (saving the offcuts for another day). Place the mushrooms cut side down on one side of a large dry non-stick frying pan on a high heat. Cook for 5 minutes while you scrunch and work the mince with your hands. Divide into two equal balls, flatten each to just under ½cm thick, then pat and push a rasher of bacon on to each one.
Turn the mushrooms, put the burgers into the pan, bacon side down, sprinkle with a pinch of sea salt and black pepper, and fry hard and fast for 2 minutes, pressing down with a fish slice to crisp up the bacon, then flip to fry for just 1 minute on the other side. Move the mushrooms on top of the burgers, then halve the bun and quickly toast alongside. Remove the bun to a serving board, spread with the soured cream, stack in your burgers and mushrooms, then slice and add the gherkin.
Off the heat, coarsely grate the cheese in a pile at the cleanest side of the pan, pour the Worcestershire sauce on top, then tilt the pan and stir vigorously for 30 seconds until it combines into an oozy consistency. Pour the rarebit mixture over the burger stack and put the bun lid on. Devour.