Preheat the oven to 400°F. Place one oven grate at the bottom and one in the middle of the oven.
Spray a baking dish or roasting pan, at least 9 by 13 inches, with cooking spray.
Transfer the sliced potatoes, onion and red pepper to your roasting pan or baking dish.
Add the olive oil and toss together for a few minutes ensuring all are well coated with the oil. Arrange in a single layer.
Cover with a piece of aluminum foil and bake on the bottom oven grate for 30 minutes.
While the potatoes and vegetables are roasting in the oven, let's prepare the marinade.
In a small bowl, whisk together the balsamic vinegar, dried thyme, salt and freshly ground pepper. Set aside for now.
After 30 minutes, increase the oven temperature to 450°F.
Remove the foil, add the balsamic marinade and toss gently together. Ensure the potatoes have the cut side down.
Return the dish to the oven and place it on the middle grate. Roast for approximately 30-40 minutes or until the potatoes are golden and crispy on the edges. Gently toss the vegetables a few times as they are roasting in the oven.
Remove from the oven, transfer to a serving dish and garnish with the chopped fresh flat-leaf Italian parsley.
