Step 1

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.

Step 2

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.

Step 3

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.

Step 4

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.