Deuteronomy 12:20, with Rabbi Kaplan's translation:
When God expands your borders as He promised you, and your natural desire to eat meat asserts itself, so that you say, 'I wish to eat meat,' you may eat as much meat as you wish.
That last "you may eat" is a translation of "תֹּאכַל", which can also be translated "you shall eat". Someone told me recently that Rav Achay Gaon says that this is one of the 613 mitzvos (commands): if someone wants to eat meat, he fulfills a mitzva by doing so. I don't see it in the Sh'iltos ad loc.; does anyone have a citation?