Cut off the top and bottom of each citrus fruit so it can sit flat on the cutting board. Stand the fruit upright and use a sharp knife to slice downward, removing the peel and white pith. Rotate and repeat until fully peeled. Discard peels. Turn the peeled citrus on its side and slice into ¼-inch-thick rounds.

Cover the sliced citrus and refrigerate for up to 24 hours before serving.

In a mixing bowl, whisk together lemon juice, extra virgin olive oil, Dijon mustard, honey, sea salt, and black pepper until the dressing is smooth and creamy. Taste and adjust the seasoning if necessary. Cover and refrigerate; the dressing can be kept for up to 3 days.

On a serving plate or bowl, spread the arugula as the base, arrange the citrus slices evenly over it, scatter thinly sliced red onions, and sprinkle with walnuts for crunch.

Drizzle the prepared dressing over the salad just before serving. Store any leftovers in an airtight container and consume within 2 days for best quality.