Good Q! I'm not 100% sure on the spinach conversion. Whenever I use frozen spinach I just use 1 cup vs. 2 cups. The consistency may be a bit different, however it tastes about the same. For Butterhead lettuce, maybe this will help? http://www.gourmetsleuth.com/Dictionary/B/Bibb-lettuce-5234.aspx If not, any other leafy green of your choice should work!