Preheat your oven to 400°F (200°C).
Wash and trim the beetroots, leaving the skins on.
Wrap each beetroot in aluminum foil and place them on a baking tray.
Roast for about 45 minutes, or until tender when pierced with a fork.
Remove from the oven and let them cool for a few minutes.
Peel the skins off the beetroots and slice them thinly.
In a large mixing bowl, combine the salad greens and sliced beetroots.
Add the crumbled feta cheese and chopped walnuts to the bowl.
Drizzle with olive oil and balsamic vinegar, then season with salt and pepper.
Toss gently to combine all ingredients and serve immediately.