Below are some alternatives which may be used to improve the sentence by replacing the word(s) in bold. You may choose option 1, 2, 3 or 4. In case, no improvement is needed, choose option 5.
First of all, the sentence describes an action which had taken place in the past.
Also, Robinson Crusoe sat down after the meal was ready and not while the meal was being cooked.
Hence, the continuous tense i.e 'is being' should be replaced by 'had been'.