How to Make It
In a large saucepan, cover the potato rounds with water and bring to a boil. Add a generous pinch of salt and simmer over moderate heat until the potatoes are tender but not falling apart, about 12 minutes. Drain and transfer to a baking sheet to cool.
Light a grill or preheat a grill pan. Lay 2 large sheets of heavy-duty foil on a work surface. Fold up the edges to form a 1/2-inch rim and pinch the corners together to seal. Transfer the foil to the grill and melt the butter on it. Add the potatoes in a single layer and grill over moderate heat until browned on the bottom, 12 to 15 minutes.
Meanwhile, in a small saucepan, bring the rice vinegar just to a boil. Add the mustard seeds and let cool completely. Whisk in the sherry vinegar, maple syrup, lemon juice and rice bran oil. Season the dressing with salt and pepper.
Transfer the potatoes to a large bowl. Add the celery, scallions and dressing and gently toss to coat. Season with salt and pepper and toss again. Serve right away.