Lay peppers on their sides on racks of gas burners and turn flames on moderately high. Roast, turning frequently with tongs, until skins are blackened, 5 to 8 minutes. (Or broil on rack of a broiler pan about 4 inches from heat, turning frequently, 15 to 20 minutes.)
Transfer to a bowl, then cover, and let steam 10 minutes. Alternatively, place peppers in re-sealable plastic bags to steam.
Peel peppers over bowl (to catch any liquid) and discard stems and seeds. Cut peppers lengthwise into 1-inch thick strips and add to liquid in bowl with vinegar, thyme, and capers.
Add salt and pepper, to taste, and toss. Divide arugula among 10 plates. Top with peppers and drizzle with any remaining dressing. Serve at room temperature or chilled.
Cooks' note: Dressed peppers can be made 3 days ahead and chilled, covered;
Recipe courtesy of Gourmet Magazine