Cook the pasta according to the directions on the package. Drain the pasta, rinse it under cold water, and put it in the refrigerator to chill.
In a dry saute pan toast the walnuts over a medium-high flame until they are fragrant, about 2 minutes. Set the walnuts aside to cool then chop them coarsely.
In a large bowl, toss together the chilled pasta, walnuts, feta cheese, onion, and spinach. In a small bowl whisk the oil, vinegar, garlic and mustard. Pour the dressing over the pasta salad and toss to combine. Season with salt and pepper.