Place pesto ingredients in a food processor and process until coarsely chopped and well blended. Scoop into a bowl and set aside until soup is ready. To make the soup, heat the olive oil in a pot over medium heat. Add the green onions and cook until just tender, about 3 minutes. Stir in flour. Gradually whisk in stock. Bring to a boil and cook until lightly thickened, about 5 minutes. Add the spinach and cook until it just wilts, about 1-2 minutes. Puree the soup in batches in a food processor or blender. Return to the pot. Add the milk and re-warm over low heat; season with salt and pepper. Ladle soup into bowls. Place a generous spoonful of pesto in the centre and serve.