Slice Brussels sprouts across in 2 mm strips, using a food processor or slicer. Discard cores (use for healthy snacks.)
Chop walnuts into 1 cm chunks.
Grate cheese (aged Gruyere, not processed smoked Gruyere) (or parmesan).
Toss cheese with sprouts and nuts.
Whisk together mustard, oil, vinegar, salt and pepper, until blended.
Pour over salad, toss lightly, and serve.
