To make the dressing put the mustard, walnuts, and vinegar in a small food processor. Process for about a minute, then, while the machine is running, drizzle in the oil (there should be a little hole in the top to allow the oil to drizzle slowly in.) When all the oil had been incorporated, scrape down the sides and process until everything is nice and creamy. The longer you process the smoother the consistency will be. Season with salt and pepper to taste.
Slice the pear into 1/4 inch slices. I like to use a mandolin slicer for this. Sprinkle the cut slices with the lemon juice to prevent browning, making sure all surfaces are covered.
Arrange arugula on 4 salad plates and top each pear slices, stacked but fanned out slightly. Scatter the nuts and blue cheese over the plates. Drizzle with the dressing, and season with fresh cracked black pepper.