How to Make It
In a large pot of boiling salted water, cook the orzo until al dente. Drain and transfer to a large bowl; let cool.
Meanwhile, in a medium skillet, toast the pine nuts over moderate heat, shaking the skillet, until the nuts are golden brown, about 3 minutes. Transfer to a plate. Add 2 tablespoons of the olive oil to the skillet. Add the shiitakes, season with salt and pepper and cook over moderate heat, stirring occasionally, until tender and browned, about 5 minutes. Set aside.
In a small bowl, whisk the vinegar, lemon juice and the remaining 1/2 cup of olive oil; add to the orzo to loosen it up.
Add the spinach, celery, feta, pine nuts and mushrooms to the orzo and toss well. Season with salt and pepper. Serve at room temperature or lightly chilled.