Preheat your oven to 425°F (220°C). Prepare a large sheet pan.
In a blender or food processor, combine the roughly chopped onion, olive oil, fresh parsley, and minced garlic. Purée the mixture until it forms a smooth paste.
In a large bowl, toss the sliced potatoes with the garlic-herb purée and the salt until every slice is evenly coated.
Transfer the coated potatoes to a ready-made foil oven bag or create your own packet by placing them on a large sheet of heavy-duty foil.
Arrange them in no more than two layers and crimp the edges of the foil to seal it tightly.
Place the foil packet on the sheet pan and bake on the center rack for 45 minutes, or until the potatoes are tender when pierced with a fork.
Carefully open the foil packet (watch for steam!) , season with black pepper, and serve immediately.