To roast peppers using a charcoal grill: When fire is hot (you can hold your hand 5 inches above rack for 1 to 2 seconds), grill peppers on rack, turning with tongs, until skins are blackened, 10 to 12 minutes.
To roast peppers using a gas grill: Preheat all burners on high, then adjust heat to moderately high. Grill peppers on rack, turning with tongs, until skins are blackened, 10 to 12 minutes.
Transfer peppers to a large bowl and cover tightly with plastic wrap, then let stand until cool enough to handle. Peel peppers, then halve lengthwise, discarding stems and seeds.
Stir together remaining ingredients, then add peppers and toss gently. The peppers in can be made 1 day ahead and chilled, covered. Bring to room temperature before serving.
Cook's note: If you aren't able to grill, peppers can be roasted on racks of gas burners over high heat, turning with tongs. Or broil on broiler pan about 5 inches from heat, turning occasionally (about 15 minutes).