Preheat the oven. Heat the oven to 375°F.

Prep the beets. 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 the beets. Roast for 1 to 1 1/2 hours, or until the beets are tender enough to be pierced with a fork. When the beets are cool enough to handle, slip the skins off and cut off the ends. Slice them lengthwise, then cut them into 1/4-inch slices.

Assemble the salad. Place the beets in a serving bowl. Quarter and slice the red onion. Toss the sliced onion with the sliced beets.

Make the dressing. 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.

Chill. Cover and refrigerate the beet salad until it's time to serve.