Heat a frypan over medium heat and cook bacon in batches until golden. Set aside on a paper towel to drain. Wipe off any charred bits from the pan.
Brush mushrooms with olive oil and generously season.
In the same frypan cook mushrooms 4 minutes on one side, then turn and top with a cheese slice. Cook for another 4-5 minutes allowing cheese to melt. Set aside.
Wipe the pan clean, then cook the buns cut-side down until they begin to colour.
Assemble burgers with BBQ sauce and mustard on the bun base, topped with a mushroom, bacon, pickles and vegetables. Add ranch dressing to the top bun and serve immediately with a side of hand-cut fries.