Grilled white and sweet potato salad

Ingredients:

2 large waxy (new) potatoes, about 1 pound
1 or 2 sweet potatoes, about 1 pound
4 tablespoons extra virgin olive oil
Salt and pepper
1 tablespoon grainy mustard
1 tablespoon sherry or other vinegar, or to taste
1 bunch scallions, both white and green parts, chopped.

Preparation:

Start a gas or charcoal grill, or heat the oven to 450F.
Peel potatoes and cut them into slices 1/2-inch thick.
Toss them with half the olive oil and sprinkle them with salt and pepper.
Grill potatoes over direct but not-too-hot heat, turning them as they brown. Or roast them on nonstick baking sheet, turning once or twice.
Potatoes will cook in about 10 to 15 minutes. Remove them as they become tender.
When they are done and fairly cool, toss them with remaining ingredients.
Taste and adjust seasoning and serve.
The salad can be covered and refrigerated.
Bring to room temperature before serving.