Preheat the oven to 400 degrees F. Toss the potatoes with 1 tablespoon of the olive oil and a pinch of thyme. Place in a single layer in a roasting pan and roast for 15 minutes. Meanwhile, combine the green pepper, onions, and red pepper with the remaining oil and a pinch of thyme. Add to the potatoes after 15 minutes, loosening the potatoes and stirring the mixture. Continue roasting, stirring occasionally, until the potatoes are browned and tender and the onion begins to caramelize at the edges, about 45 minutes. Spoon into a large bowl and let cool slightly.

Combine the vinegar, salt, pepper, and another pinch of thyme. Add to the potato mixture. Serve the salad warm, cold, or at room temperature.