1) In a medium pan over medium heat, cook the bacon until golden and crispy and the fat is rendered, about 3 to 5 mins. Remove with a slotted spoon, drain on paper towels, and reserve.
2) Halve the avocado, remove the seed, and scoop the halves out of the skin with a large spoon. Dice the avocado, and in a small bowl, gently toss with lemon juice.
3) To assemble the salad: use a large, straight-sided glass bowl, if possible - layer the ingredients in the following order: romaine lettuce, chicken, tomatoes, Bibb lettuce, eggs, reserved bacon, spring onions, avocado, watercress, and finally the blue cheese. Season with pepper to taste.
4) At the table, gently toss the salad with about 180-240ml of the vinaigrette. Divide the salad among chilled plates. Serve.
5) For the basic vinaigrette: In a small mixing bowl, whisk together the vinegar, mustard, 1 tsp salt, and pepper to taste. Gradually whisk in enough of the oil to make a smooth dressing with a balanced taste. Season with salt and pepper, if needed. Use immediately or store covered, in the refrigerator, for up to a week.
* Cook's Note: When making vinaigrette with a potent vinegar, like sherry, use the larger amount of oil.