For the dressing, put the oil, vinegars, mustard, sugar and salt and pepper in a bowl and whisk until evenly combined and thickened. Taste and season again if needed, then mix in the dill.
Put the spring onions, celery and fennel in a large salad bowl. Pour in the dressing and toss well.
Place half the cucumber and half the mixed leaves, Romaine lettuce and rocket in the bowl. Season, then top with the remaining cucumber, leaves, Romaine lettuce and rocket. Season well and chill for up to 4 hours