Preheat the oven to 350 degrees F. Toss the potatoes with a drizzle of olive oil, salt, and pepper. Spread on a sheet pan and roast until tender and golden brown, about 40 minutes. Shake the pan halfway through cooking so the potatoes cook evenly.
Place the onion on a large piece of aluminum foil. Drizzle the onion with olive oil and season with salt and pepper. Wrap the onion tightly in the foil and roast until tender and lightly golden brown, about 50 minutes to 1 hour. Cool slightly. When cool enough to handle, chop and set aside.
In a small bowl, whisk together the lime juice and macadamia oil. Combine the potatoes and onions in a large bowl. Add the lime juice mixture and toss well. Season, to taste, with salt and pepper.