Toast walnuts in a dry skillet over medium heat, or on a baking sheet at 180ºC for 3 minutes. Set aside. Steam spinach until just wilted. When cool enough to handle, remove excess moisture with paper towels or a salad spinner. Blend spinach, walnuts & remaining ingredients in a food processor or blender until the mixture resembles a rough paste. Serve immediately tossed with hot pasta or refrigerate in an air-tight container for up to 1 week.