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You could substitute collard greens. Also kale, and it takes longer to cook. Spinach if you are in a pinch but it is much lighter and doesn't have the heartiness of flavor.
I agree with SKK. Collard greens would be my first choice, a non-curly kale like lacinato or dinosaur kale would be my second choice. Remove the stems. If I were subbing spinach, I might use defrosted frozen spinach so that it were softer. Long leaf spinach might be okay, too.
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Yes, lacinato aka black kale and in Italy, cavolo nero would probably be the best substitute. Spinach, no.
Ditto the other respondents! Swiss chard tends to be sweeter than kale and collard greens so I would go with a bunch of leafy spinach.
Kale or beet greens.
Beet greens would be my first choice since they are basically the same thing. And then after that I'd use kale or collards.
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