Combine cilantro, spinach, and chicken stock in a blender.

Blend until finely pureed.

Add milk and salt to the blender.

Blend briefly until combined.

Set the mixture aside.

In a 3-quart saucepan with a lid, heat olive oil and butter over medium heat.

Add rice to the saucepan and sauté until it begins to brown, approximately 3 minutes.

Add minced onion and garlic to the rice and cook for another minute, stirring constantly.

Pour the blended mixture into the saucepan with the rice.

Stir well to combine.

Bring the mixture to a boil, then reduce heat to low.

Cover the saucepan with a lid.

Simmer for at least 20 minutes, or until the rice is cooked through and the liquid is absorbed.

Carefully stir the rice with a fork to fluff it.

Cover the saucepan again and let it cook for another 5 minutes.

Turn off the heat and let the rice steam, covered, for about 10 minutes before serving.