Arrange peppers on a broiler pan and broil peppers about 2 inches from heat, turning occasionally with tongs, until skins are blackened, 20 to 30 minutes. Transfer to a large bowl and cover tightly with plastic wrap, then let stand 20 minutes. When cool enough to handle, peel peppers, discarding stems and seeds, and quarter each lengthwise.
While peppers are broiling, soak raisins in 1 cup hot water 15 minutes, then drain well in a sieve. Whisk together lemon juice, harissa, and sea salt, then whisk in oil until combined well.
Toss peppers with dressing and sprinkle with raisins and walnuts.
Roasted peppers can be tossed with dressing 1 day ahead and chilled, covered. Bring to room temperature before proceeding.