Preheat the oven to 400°F and line a baking sheet with foil.
Add the potatoes to the center of the foil. Drizzle them with the oil and sprinkle with the onion, garlic and salt. Use your hands to toss together until all of the potatoes are well coated.
Spread the potatoes out so they are in one even layer and then crack a bit of black pepper over them.
Add a second piece of foil very lightly on top. Fold the edges of the bottom piece over the edges of the top piece, being sure it puffs up a bit, like a bag. (In other words, do not place the top sheet of foil directly on the potatoes -- you want a good couple of inches of space between the potatoes and the top sheet.)
Place in the preheated 400°F oven and steam the potatoes for 15 minutes. Then remove the potatoes from the oven and turn the heat up to 500°F.
Take off the top sheet of foil -- very carefully because the steam that will release will be very hot!
Once the oven has heated to 500°F, return the now uncovered potatoes. Roast them until the bottoms of them are golden brown, about 10 minutes. Gently flip them over and continue to roast until the other side is golden brown, about another 10 minutes. (During the roasting process, some of the bits of onion and garlic will likely blacken on the foil. That's okay -- just discard it when the potatoes are done.)
Taste and season with more salt if necessary.
Let them cool for about 5 minutes on the baking sheet before serving.