Preheat the oven to 350°F. Spread the walnuts out on a small baking sheet in a single layer. Bake for 8 to 10 minutes, or until the walnuts are lightly toasted. Set aside to cool.
In a large mixing bowl, add the olive oil, vinegar, maple syrup, Dijon mustard, salt, and pepper and whisk to combine. Add the apples on top of the dressing and toss with the dressing to keep it from browning.
Place the arugula and onions on top of the apples and toss again to combine with the dressing. Sprinkle the toasted walnuts, cranberries, and feta on top, and serve immediately.
