How to Make It
Preheat the oven to 400°. Wrap each beet in foil and bake them directly on an oven rack for 1 1/2 hours, or until tender. Let cool, then peel and cut into 1/2-inch wedges.
Meanwhile, in a large baking dish, toss the carrots with the sugar and 2 tablespoons of the olive oil. Season with salt and pepper. Roast the carrots for about 45 minutes, tossing occasionally, until they are tender and browned in spots.
Meanwhile, in a large saucepan, combine the lentils with the water, onion, coriander, bay leaf and 2 whole garlic cloves and bring to a boil. Reduce the heat to moderate and cook until the lentils are barely tender, about 20 minutes. Add 2 teaspoons of salt and cook until the lentils are tender, about 5 minutes longer. Drain; discard the onion, garlic cloves and bay leaf.
Transfer the lentils to a large bowl. Stir in the minced garlic, preserved lemon peel, lemon zest, lemon juice, mint, shallot and the remaining 1/2 cup of olive oil. Season with salt and pepper.
In another large bowl, toss the beets and carrots with the balsamic vinegar and arrange on the lentils. Sprinkle the cilantro and scallions on top and serve warm or at room temperature.
Black lentils are available at Indian and Middle Eastern markets. Jarred preserved lemons are available at specialty shops and Middle Eastern markets.