Bring a large pot of salted water to a boil and cook the haricot vert until al dente, about 90 seconds. Shock in ice water, drain well, chop in halves or thirds and reserve.
Put the potatoes in a large pot, cover with 2 inches cold water and add 1 tablespoon salt. Bring to a boil and cook until a small knife inserted into the center of the potato meets with some resistance, about 5 minutes. Drain well, place on a baking sheet and let cool slightly. Halve lengthwise.
Whisk the vinegar, lemon juice, mustard, honey and salt and pepper. Slowly whisk in the olive oil until emulsified.
Heat the grill to medium heat. Toss the potatoes with canola oil and sprinkle with salt and pepper. Grill until golden brown in spots, tossing occasionally, about 5 minutes. Remove and toss with some of the vinaigrette.
Combine the potatoes, green beans, feta cheese, olives and oregano in a bowl. Toss with more of the vinaigrette, taste and season with salt and pepper if necessary. Scatter the parsley leaves over the top and drizzle with a bit more olive oil.