Make the dressing. Mix the vinegar, sugar and mustard in a bowl. Add a dash of salt and pepper, then slowly whisk in the olive oil. Stir the chopped walnuts and herbs into the dressing and add more salt and pepper if needed.
Chop the avocado into cubes and sprinkle with lemon juice to stop it going brown.
Put the spinach leaves in a bowl together with the spring onion strips and avocado cubes.
Heat the oil in a frying pan and fry the bacon and the garlic until crisp and brown, then drain on kitchen paper. Scatter over the spinach mixture.
Drizzle some of the dressing over the salad, toss gently and serve straight away, garnished with extra walnut pieces, if liked.