Using a blender, combine the mustard, salt and pepper and lemon juice. Start adding in olive oil until you reach desired constancy. Season with salt and pepper to taste. Set dressing aside while you complete the salad.
Toss thinkly sliced brussel sprouts, apple and walnuts together in a large bowl.
Pour the dressing over the salad mix and stir to coat well. Top with cheese.
Dress this salad two to three hours in advance for best results. It allows time for the brussels sprouts to marinade in the dressing for more flavor.
After dressing, you can keep the salad leftovers in the fridge for up to two days without it becoming soggy.
Salad is also excellent with dried cranberries or cherries added.