Preheat the oven to 400 degrees.
Spread the potatoes in an even layer on a baking sheet.
Drizzle 1 tablespoon of olive oil over the top of the potatoes and season them with a few pinches each of salt and pepper.
Stir the potatoes around to coat them evenly in the oil.
Roast the potatoes for 20 to 25 minutes, or until they are fork tender.
Remove them from the oven and cool them in the pan while you prepare the rest of the salad.
In a small bowl, whisk the mustard and lemon juice with 3 tablespoons of olive oil until combined.
Season the dressing with a generous pinch of salt and a pinch of pepper.
In a large bowl, toss the arugula, green onion, parsley, and potatoes together.
Add two tablespoons of the dressing to the bowl and gently toss it with the greens and potatoes until they are evenly coated.
Divide the salad between two plates and garnish with the walnuts and feta cheese.
