Leave the lettuce leaves whole if small, or shred as finely as you like. Peel and slice the onion. Put the lettuce and onion into a salad bowl, add the vinaigrette and lightly turn the lettuce over in the dressing, until well mixed.
Other salad vegetables can be added:
Beetroot (cooked if necessary, and sliced).
Celery (washed, scraped if necessary, cut into 1in lengths).
Cucumber (washed, cut into rings or chunks).
Pepper (washed, cored, cut into rings).
Radishes (with tops cut off, roots removed, and washed).
Spring onion (washed, cut off roots and yellow leaves, cut into rings or leave whole).
Tomatoes (washed, sliced or cut into quarters).
Watercress, mustard and cress (washed, sprinkled on top of the other vegetables).