We have prepared the meal itself: the database. We have done the bulk of the job and in principle met (almost) all our objectives for the course. 

Now we can engage in the preparation of a side-dish: the gravy. The database itself is a working product, ready for use, just like a plate of boiled potatoes is ready for use. 

But after the easy step of upsizing it, it will be even better, just like a plate of potatoes will be better with a ladle or two of gravy poured atop of it. 

We can rephrase

> Now for the gravy... 

as

> Now let's see what additional job we can do to make the database better.