1. In a non-stick frying pan over low heat, heat 1/4 cup oil. Add leek and sauté until soft and caramelized. Add garlic, spinach, and salt, and stir to cook through.
2. Transfer mixture to a tall, deep container and add nuts and water. Blend with immersion blender until smooth. In a bowl, combine pesto with chicken, stir to coat, and allow to marinate for 20 minutes.
3. In a frying pan, heat a little oil. Add chicken and fry until cooked through, turning once in middle. Repeat with remaining chicken and serve immediately.
If you prefer not to fry, you can prepare this chicken on a grill or in the oven, with equally delicious results!