Cover the potatoes in salted water and bring to a boil.
Boil for 8-10 minutes, until you can stick them with a fork, but they are still a bit firm.
When cool enough to handle, slice potatoes in half lengthwise.
Toss the potatoes with 2 tablespoons of olive oil, salt, and pepper.
Rub the onion slices and pepper with 1 tablespoon of olive oil.
Place the pepper halves directly on the grill and place the potatoes and onion in a grill basket over a medium-hot flame for about 20-30 minutes. Stir/flip occasionally.
While the other ingredients are grilling, make your dressing. Whisk together the remaining 3 tablespoons of olive oil, the juice and zest of 1 lemon, vinegar, mustard, and parsley. Season to taste with salt and pepper.
Dice the grilled onion and pepper.
Toss the vegetables together with the dressing and serve.