Scrub the beets. Trim the ends, leaving about 1 inch of stem and root. Sprinkle the beets with salt and black pepper and then drizzle 1 tablespoon of the olive oil over them. Wrap the beets tightly in foil and place them on a rimmed baking sheet or pan.
Roast for 1 to 1 1/2 hours, or until the beets are fork-tender.
When the beets are cool enough to handle, slip the skins off and cut off the ends. Slice them lengthwise and then slice them into 1/4-inch slices. Put the beets in a serving bowl.
Quarter and slice the red onion. Toss the sliced onion with the sliced beets.
In a cup or bowl, combine the lemon juice, zest, and mustard; add the remaining 3 tablespoons of olive oil in a thin stream, whisking constantly. Add the dressing to the beets along with freshly ground black pepper and salt, to taste. Add the cilantro and toss gently to blend.
Cover and refrigerate the beet salad until it’s time to serve.