Preheat the oven to 400 degrees F (200 degrees/Gas 6).
Bring a large pan of water to the boil, add the potatoes, and parboil them for 3 to 4 minutes. Drain and set aside. Heat some olive oil in a pan and fry the onions gently with the thyme leaves until meltingly soft. Add the balsamic vinegar and remove from heat. Remove onions to a plate.
Using the same pan (no need to wash it), lay half the potatoes in the bottom and season with salt and pepper. Follow with the onions in 1 layer, seasoning again, then the rest of the potatoes. Bake in the preheated oven for about 45 minutes.