Heat the oil and butter in a large heavy bottomed pan. Add the onions and cook on a really low heat, uncovered for 40 minutes or until the onions are sticky and caramelized. Pour over the ale and reduce by half. Add the stock and bring to the boil. Cook for 30 minutes.
Heat the grill on high. To make the Welsh rarebit toasts, mix together the cheese, mustard, egg, Worcestershire sauce, ale and salt and pepper. Grill the slices of bread on one side. Sprinkle the cheese mixture between each of the toasts on the un-toasted side.
Pour the soups between 4 bowls. Lay 2 of the rarebits on top of each of the bowls of soup then grill for 1-2 minutes or until the cheese is melting. Serve the soups immediately