Warm Winter Vegetable Salad

1 small red onion, cut into 1/2-inch wedges
1 small sweet potato, cut into 1-inch pieces
1 carrot, peeled and cut into 3/4-inch rounds
1 parsnip, peeled and cut into 3/4-inch pieces
1 small celery root, peeled and cut into 3/4-inch pieces
1 small beet, peeled and cut into 3/4-inch pieces
3 tablespoons extra virgin olive oil
Salt and freshly ground pepper
1/4 cup walnuts
1 and 1/2 teaspoons balsamic vinegar
1 and 1/2 teaspoons fresh lemon juice
1/2 teaspoon Dijon mustard
2 tablespoons flat-leaf parsley
1/4 cup crumbled feta cheese