A large number of recipes are seasonal, whether tied to specific holidays or only popular at certain times of year.
Search traffic for strawberry rhubarb is non existent in December, and gingerbread cookie recipes don't get traffic in July.
You can find your previous years seasonal recipes by visiting Google Search Console "Performance" page and setting the custom date range to last year.
For example, for June 2021, this would be 2020-06-01 to 2020-06-30.
The post IDs are then used in the FSRI block specific posts field.
New sites won't have any historical seasonal content and shouldn't worry about this optimization.
Remember that it takes a new post 6-9 months to fully rank on existing sites, and new sites won't rank at all for 9-12 months, so your only focus during the first year is putting out as much high quality recipe content as possible.