Begin by preparing the beets. You may either boil or roast them.

To boil, place the unpeeled beets in a saucepan and cover them with water. Bring to a boil, then reduce heat and simmer for approximately 30 minutes, or until a fork easily pierces the beets.

To roast, preheat the oven to 400°F (200°C). Wrap the unpeeled beets individually in aluminum foil and place them on a baking sheet. Roast for about 1 hour, or until tender when pierced with a knife.

Once cooked, allow the beets to cool to room temperature. Gently remove the skins by rubbing them with paper towels, then slice or quarter the beets as desired. If time permits, marinate the sliced beets in half of the vinaigrette for 10–15 minutes to enhance flavor.

While the beets are cooking, prepare the dressing. In a small jar or bowl, combine the olive oil, white wine vinegar, dried mustard, salt, and black pepper. Seal the jar and shake vigorously until the vinaigrette is well blended and emulsified. Adjust seasoning as needed.

On individual serving plates or a large platter, arrange a layer of arugula. Distribute the sliced beets and oranges evenly over the greens. Add several slices of red onion and sprinkle with chopped walnuts, if using.

If desired, for additional color, place a few orange slices in the reserved beet juice for several minutes before arranging them on the salad. This will give the oranges a subtle pink hue.

Finally, drizzle the prepared vinaigrette evenly over the salad just before serving. Serve immediately.