Melt the butter in a large stock pot over a low heat and then stir in the bay leaves, onions, leeks and garlic. Season with a generous pinch of sea salt and several grinds of fresh black pepper, then leave everything to cook down slowly, stirring occasionally, until the vegetables are softened almost to the point of collapse, and significantly reduced in volume. This will take up to an hour.
Sprinkle over and stir in the flour, then add the cider and stock. Turn up the heat and bring to a gentle simmer. Let the soup putter away for 15 minutes or so, until it has thickened up.
To finish add the raw cider vinegar a little bit at a time, tasting as you go (be careful not to add too much will ruin the soup, but the right amount will freshen it beautifully). Season with a little more pepper and salt to taste.
To serve, ladle the piping hot soup into deep bowls. Toasted sourdough with mature cheddar cheese and a sprinkling of Worcestershire sauce is a very fine accompaniment.