Preheat the oven to 400°F (200°C).
Wash and trim the beets, then wrap them in aluminum foil.
Roast the wrapped beets in the oven for about 45 minutes until tender.
Once cooked, let the beets cool before peeling them.
Slice the peeled beets into rounds or wedges.
In a large bowl, combine the mixed greens and sliced beets.
In a small bowl, whisk together olive oil, balsamic vinegar, salt, and pepper.
Drizzle the dressing over the salad and toss gently.
Add crumbled feta cheese and chopped walnuts on top.
Garnish with fresh herbs if desired, and serve immediately.