Toss potatoes, 1 tablespoon olive oil, and salt together in a 9x13-inch baking dish until coated.
Bake until the potatoes are easily pierced with a fork, about 35 minutes.
Meanwhile, bring lentils, water, and salt to a boil in a saucepan. Reduce heat to medium-low and simmer until lentils are tender, about 20 minutes. Drain. Combine potatoes, lentils, red bell pepper, and green onions in a large bowl.
Whisk yogurt, Dijon mustard, cider vinegar, 2 tablespoons olive oil, and turbinado sugar together; drizzle over potato mixture and toss to combine. Season with salt and black pepper.