The Simpsons episode of interest is called ‘Lisa the Vegetarian’, which was the fifth episode in season seven originally. It first aired in the US in October, 1995.
From Wikipedia: In the episode, Lisa decides to stop eating meat after bonding with a lamb at a petting zoo. Her schoolmates and family members ridicule her for her beliefs, but with the help of Apu, Paul, and Linda McCartney, she commits to vegetarianism. The pair’s condition for appearing was that Lisa would remain a vegetarian for the rest of the series. The episode makes several references to McCartney’s musical career, and his song ‘Maybe I’m Amazed’ plays during the closing credits.
During ‘Lisa the Vegetarian’, Paul says, “If you play ‘Maybe I’m Amazed’ backwards, you’ll hear a recipe for a really ripping lentil soup.” The song is played during the closing credits, and Paul’s reading of the recipe can be heard if the sound is played backwards.
The recited recipe can be heard (played forwards) in the extra features on The Complete Seventh Season DVD boxset.