Preheat the oven to 450°. Wrap the beets in foil and roast for about 1 hour, until tender. When cool enough to handle, slip off the skins and cut the beets into wedges. Cut the beets into quarters.
Meanwhile, in a medium saucepan of boiling water, cook the lentils until tender, about 20 minutes. Drain and cool under running water; drain well.
Whisk together, yogurt, mint, lemon and salt. Toss beets with lentils and spinach, plate and drizzle with yogurt mint sauce.