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Preheat the oven to 350°F (180°C).
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Cut off the top of the head of garlic, wrap the bulb in aluminium foil, and roast it on a baking sheet for 30 to 40 minutes, until tender. Allow the bulb to cool and then squeeze the garlic pulp out of the husks. This should yield 2 tablespoons or more of roasted garlic. Set aside.
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Heat the oil over medium-high heat in a large saucepan. Add the green onions, red onion, and 1/4 teaspoon of salt and sauté for 5 minutes, until the onions are translucent. Add the celery, zucchini, and parsley and sauté for 5 minutes more.
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Stir in the roasted garlic and the vegetable broth. Increase the heat to high and bring just to a boil. Reduce the heat to medium and simmer for 10 minutes. Add the spinach and simmer for 5 minutes more, until the leaves are just tender.
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Remove the saucepan from the heat and allow the soup to cool somewhat. Stir in the nuts.
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Pour the soup into your blender in batches, and puree on high for 30 to 60 seconds until smooth and creamy.
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Return the soup to the saucepan and warm over medium-low heat. Season to taste with salt and pepper and serve.