Rinse beets. Wrap colors separately in foil and roast in middle of oven until tender, 45 minutes to 1 hour. Cool.
Peel beets and halve lengthwise. If using medium beets, cut each half crosswise into 1/4-inch-thick slices, keeping colors separate.
Whisk together shallot, juices, zest, and mustard. Whisk in oil until emulsified and season with salt and pepper. Drizzle lettuce with three fourths of dressing. Sprinkle with beets and drizzle with remaining dressing.
Each serving has about 61 calories and 4 grams fat. Nutritional analysis provided by Gourmet