Preheat the oven to 200°C (400°F) if roasting beetroots.
Wash and trim the beetroots, then wrap them in foil.
Roast beetroots for about 45-60 minutes until tender.
Allow the beetroots to cool, then peel and slice them.
In a large bowl, combine the mixed salad greens and red onion.
Add the sliced beetroots to the bowl.
Drizzle olive oil and balsamic vinegar over the salad.
Sprinkle crumbled feta cheese and chopped walnuts on top.
Season with salt and pepper.
Toss gently to combine all ingredients and serve immediately.