6 medium beets, trimmed
1 large English hothouse cucumber, unpeeled, cut into 1/4-inch-thick rounds
1/2 sweet onion (such as Maui or Vidalia), halved lengthwise, thinly sliced
4 teaspoons honey
4 teaspoons apple cider vinegar
1 1/2 teaspoons Dijon mustard
1/3 cup corn oil
1. Preheat oven to 400°F. Wrap each beet in foil, enclosing completely. Place on rack in oven and bake until beets are tender when pierced with fork, about 1 hour 30 minutes. Cool in foil. Peel beets, then cut each into 6 slices.
2. Arrange beets, slightly overlapping, on half of large platter. Arrange cucumber slices on other half. Scatter onion in center. (Can be made 4 hours ahead; chill.)
3. Whisk honey, vinegar, and mustard in small bowl to blend. Gradually whisk in oil. Season with salt and pepper. Drizzle over vegetables.