If we can't find fresh turmeric, can powdered be substituted? If so, how much?

Yes, I certainly would. I'd use a heaping teaspoon, but would also finely chop some fresh ginger, if I had it - about 2 teaspoons chopped - to give the sauce a bit more zing. I have no idea whether doing that would make it totally inauthentic, but I tend not to worry about such things.

This sounds like an excellent recipe - especially since locally grown kale is plentiful here, so I'm looking for fresh new ways to use it.