Preheat the oven to 220C/200C Fan/Gas 7.
For the bacon bits, heat the oil in a large frying pan and fry the bacon until crisp and brown. Do this quite slowly over a low–medium heat so the bacon does not burn. Transfer to kitchen paper to drain and cool slightly. Chop the bacon as finely as possible and spread out on a baking tray. Cook in the oven for 5 minutes to help the bacon dry out. Remove and leave to cool.
For the salad, put the red onion in a small bowl and sprinkle with plenty of salt. Add the cider vinegar and a tablespoon of water. Leave to stand until the salad is ready to serve.
Prepare the iceberg lettuce by trimming a sliver off the stem (just to remove any brown) and then cut through the stem right down the middle. Cut each half through the stem into three wedges. Arrange on a serving plate.
For the dressing, mash the blue cheese and whisk in the remaining ingredients along with plenty of seasoning. Taste and adjust the vinegar or sugar if necessary.
To assemble, drain the onion and sprinkle over the iceberg. Drizzle with the blue cheese dressing and top with the bacon bits. Serve immediately.