Heat butter and oil in a large frypan. Add onion and fry until brown and caramelised. Pour in the stock and beer, season with salt and pepper and simmer for 5 minutes. Spray slices of French bread with olive oil and toast both sides under a grill until golden. Place 2–3 slices of bread in each soup bowl or ramekin. Pour hot soup on top, then sprinkle with crumbled stilton. Place under grill until cheese bubbles and starts to brown (about 2 minutes). Sprinkle with chopped parsley and serve immediately.