Sorry, what is a substitute for spinach, not arugula?

I too (like Creamtea) would use Kale. Actually, it may even be better than Spinach.

You could add kale-- earlier in the course of preparation since it's not as tender a leaf as spinach (I'd say, at the time you add the tomatoes). Or, simply leave it out.

You can’t get spinach or don’t like it...? For the pistou portion that needs several cups of spinach you could swap in something like swiss chard, just remove the stems first.

You actually could use arugala for the pistou, but that would have a very specific flavor so you would have to be an arugala enthusiast