Rashi on Deuteronomy 12:20/רש"י על דברים י״ב:כ׳

1כי ירחיב וגו׳ WHEN [THE LORD THY GOD] SHALL ENLARGE [YOUR BOUNDARY … AND YOU SHALL SAY, I WILL EAT FLESH … YOU MAY EAT FLESH] — The Torah teaches the proper rule of life — that one should not desire to eat flesh unless he lives amidst abundance and wealth (cf. Sifrei Devarim 75:5; Chullin 84a; see also Rashi on Leviticus 17:13).

2בכל אות נפשך וגו׳ [YOU MAY EAT FLESH] IN EVERY LONGING OF YOUR SOUL — In the wilderness, however, the flesh of a non-holy animal was forbidden to them as food, unless one first dedicated it to the altar and offered it as a peace-offering (Sifrei Devarim 75:4; Chullin 16b).