Place the spinach, cilantro, chicken broth, milk and salt in a blender container and blend for about 30 seconds, until well pureed.
In a medium saucepan heat olive oil and butter until butter is melted. Add the rice and sauté, stirring about every 30 seconds, until light golden, 3 to 4 minutes.
Add the onion and garlic and cook 1 minute, stirring frequently.
Add the spinach/cilantro mixture, stir well and bring to a boil. Cover the pan, turn heat to very low and cook for 20 minutes.
Stir the rice gently (to avoid crushing) with a fork. Cover and continue cooking on very low heat another 5 minutes.
Take the pan off heat and let rice steam in the covered pot for 10 minutes. Fluff with a fork and serve.