Peel the potatoes. Cut into quarters, lengthwise. Steam in the vegetable steamer for 10 minutes.
Cool, brush with oil. Mix together the salt and cayenne and rub onto the potatoes. Grill over a hot grill until fork tender and edges are browned. Cool and cut into cubes.
Whisk together the olive oil, mustard, vinegar and salt. Pour over the potatoes. Add the green onions, thyme and parsley. Toss.
Best served at room temperature.