Cook the orzo:
Bring a large pot of salted water to a boil. Add the orzo and cook uncovered on high for 8 to 10 minutes until al dente. Drain and rinse with cold water to cool.

Toast the pine nuts:
In a dry skillet over medium heat, toast pine nuts, stirring often, until lightly browned. Remove from heat immediately to avoid burning.

Make spinach purée:
Blend half of the spinach with 1 tablespoon of olive oil until smooth. In a large bowl, toss the cooled orzo with this spinach mixture until evenly coated.

Combine salad ingredients:
Roughly chop the remaining spinach. Add it to the bowl with the orzo, along with chopped red onion, crumbled feta, toasted pine nuts, and olives. Mix gently.

Make the dressing:
In a small jar or bowl, mix the remaining olive oil, balsamic vinegar, lemon juice, Dijon mustard, dried basil, and tarragon. Shake or whisk until well combined.

Dress the salad:
Pour the dressing over the salad and mix gently to combine all ingredients evenly.

Chill and serve:
Refrigerate the salad for at least 1 hour before serving to allow flavors to meld.