Preheat the oven to 400°F (200°C).

Toss the beets with olive oil, salt, and pepper, then spread them on a baking sheet.

Roast the beets for 45 minutes or until tender.

In a large bowl, combine mixed greens, orange segments, grapefruit segments, feta cheese, and walnuts.

In a small bowl, whisk together apple cider vinegar, Dijon mustard, honey, salt, and pepper to make the vinaigrette.

Once the beets are roasted, let them cool for a few minutes, then add them to the salad mixture.

Drizzle the mustard vinaigrette over the salad and toss gently to combine.