This is a pun on the word "beef". "Beef" is meat from cows that can be made into gravy or eaten with gravy poured on top. The phrase "have beef" is an idiom that means to be angry or annoyed at someone or something. The gravy still had lumps of beef in it. The person was bothered by the gravy's lumpiness so he "had beef" with it. 
When it was lumpy he had a beef with his gravy .