1smallred onion , halved lengthwise, then very thinly sliced crosswise
3/4lbarugula , tough stemsdiscarded and leaves cut crosswise into 1/3-inch-wide strips (6 cups)
Mince garlic and mash to a paste with a pinch of salt, then transfer to a large bowl. Whisk in oil, vinegar, salt, pepper, and sugar.
Roast bell peppers on racks of gas burners over high heat, turning with tongs, until skins are blackened, 12 to 15 minutes. (Or broil peppers on a broiler pan about 5 inches from heat, turning occasionally, about 15 minutes.)
Transfer to a bowl and cover tightly with plastic wrap, then let stand 20 minutes.
When cool enough to handle, peel peppers, discarding stems and seeds. Cut peppers lengthwise into 1-inch-wide strips, then cut strips crosswise into 1/4-inch-wide pieces.
Add peppers to dressing along with onion and arugula and toss to coat.