Preheat the oven to 400F. Rub all of the beets with a light coating of olive oil. Wrap each beet tightly in foil or parchment. Roast the beets for 45 minutes-1 hour, or until they're buttery tender when pierced with a knife. When the beets have cooled for about fifteen minutes, run them under cold water while you slip off their skins. The skins should come off easily. Chop the roasted, skinned beets into pieces that are about 1" (or a little smaller).
While the beets are roasting, bring a pot of water to boil. Drain the lentils of their soak water, then add them to the pot. Reduce the heat to medium low and simmer the lentils for 25-30 minutes, or until they're tender but still hold their shape. Drain the lentils, taking care to remove as much cooking water as possible so that they're not watery when you add them to the salad.
In a small bowl, whisk together the dressing ingredients.
Add the prepared beets, lentils, and dressing to a large mixing bowl. Toss to combine. Add the baby arugula and chives, if using. Toss again, gently. Serve the lentil beet salad with your favorite toast or bread.