Preheat the oven to 425°F (220°C).

Peel and chop the red onion, sweet potato, carrots, parsnips, celeriac, and beets into wedges.

Place the chopped vegetables in a roasting pan.

Season the vegetables with salt and pepper.

Drizzle the vegetables with olive oil.

Roast the vegetables for approximately 30 minutes, turning them occasionally to ensure even cooking.

Five minutes before the vegetables are done, spread the chopped walnuts on a cookie sheet.

Place the cookie sheet in the oven to toast the walnuts.

Toast the walnuts for about 5 minutes, or until lightly browned and fragrant.

Transfer the roasted vegetables to a serving dish.

Sprinkle the crumbled feta cheese over the roasted vegetables.

In a small bowl, whisk together 2 tablespoons of olive oil, chopped fresh parsley, balsamic vinegar, Dijon mustard, and a squeeze of lemon juice (optional) to make the dressing.

Pour the dressing over the vegetables.

Sprinkle the warm toasted walnuts over the salad.

Serve immediately.