Heat a stainless steel pan over low heat. Add the walnuts and toast them for 3 to 5 minutes, stirring now and then so they do not burn. You can skip this step and use raw walnuts, the toasting just gives a stronger flavor.
In a small bowl, mix the olive oil, balsamic vinegar, whole grain mustard and honey. Add salt and black pepper to taste.
Slice the red onion into thin rings.
Cut the apple into 4 parts, then slice each part thinly.
Add the arugula, apple, onion, prosciutto and walnuts to a large bowl. Crumble the feta over the top.
Pour the dressing over the salad and toss gently. Serve right away.
