2 1/4 cups orzo (15 ounces)
3 tablespoons pine nuts
1/3 cup plus 2 tablespoons extra-virgin olive oil
6 ounces shiitake mushrooms, stemmed, caps cut into 1-inch pieces
Salt and freshly ground pepper
1/4 cup red wine vinegar
2 tablespoons fresh lemon juice
2 cups spinach leaves (1 1/2 ounces), cut into thin strips
2 celery ribs, cut into 1/4-inch dice
9 ounces feta cheese, crumbled (1 1/2 cups)