I have a recipe that calls for toasted chopped pistachios. I'm wondering if I should toast them whole and then chop them up, or toast them chopped?
I don't think it makes any tremendous difference. I'd probably lean towards toasting them whole first, then chopping just because the size will be more consistent so the toast might be a little more even.
Toast the nuts evenly first, it will let off the steam in the nut. Then chop them. If they are evenly toasted the toasty flavor & crunch will be present throughout the nut. You will notice the difference in the nut as you eat it. :) I learned this from Christopher Kimball on America's Test Kitchen.
I would chop first as then each piece will be toasted on all it's exposed edges. If you toast then chop, the chopping will expose so untoasted edges so the overall effect will be less uniform.