I would like to substitute out the potatoes in Garbure. Which would be better sweet potatoes or butternut squash?
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Either vegetable would be a suitable substitute, just pick one based on your own preference.
Garbure is a soup that only really has three ingredients that make it Garbure - Root vegetables, white beans and cabbage. Everything else in it is dependent on what the cook has to hand, can afford to use and likes to use. Bacon, ham, duck confit, chicken, carrots, leeks, onions, turnips, parsnips, wine, stock, beer, spring water, yesterdays soup. All can go in.
My own personal preference would be to use the Squash as Sweet Potato could make it overwhelmingly sweet for me, and Garbure is a soup I like salty savoury.