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Heat the oil in a large pot like a stock pot or Dutch oven. Add the onions, celery and carrots. Sauté for about 8-10 minutes, until the veggies have softened and released some liquid.
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Add the sweet potato chunks and the water, along with the salt and whole spices. Bring to a boil, and then lower the heat to maintain a gentle simmer. Simmer until the sweet potato chunks are soft, about 30 minutes.
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Fish out the peppercorns and cloves if they will bother you (I tend to leave stuff like that in). Ideally using an immersion blender, puree the stock smooth (without an immersion blender you can either use a regular blender, or, in a pinch just mash the sweet potatoes against the side of the pot and leave the rest chunky).
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The stock can be refrigerated for 5 days or frozen for 2 months according to Caroline Randall.