Cook the medium beets until tender, about 30–40 minutes. Cool, peel, slice, and set aside.
In a large bowl, add mixed salad greens and gently toss them to fluff.
Add the sliced beets and fresh blueberries over the salad greens.
Sprinkle in the crumbled feta cheese and chopped walnuts or pecans, if using.
In a small bowl, whisk together olive oil, balsamic vinegar, and Dijon mustard or honey. Season with salt and pepper.
Drizzle the dressing over the salad while gently tossing to coat. Let sit for 10 minutes before serving.
Notes
For best flavor, serve slightly chilled. Store leftovers in an airtight container for up to 2 days. Freezing not recommended for the assembled salad, but cooked beets can be frozen separately for up to 3 months.