In a medium saucepan of boiling salted water, cook lentils until crisp-tender, about 15 minutes. Drain, rinse under cold water to stop further cooking, and drain again.
Meanwhile, in a medium bowl whisk together lemon zest, lemon juice, oil, mustard, and tarragon. Add drained lentils, bell peppers, and scallions; season well with salt and pepper. Serve at room temperature or chilled.