What was Israel told to season every grain offering with?

Answer: Salt

Leviticus 2:13: season each grain offering with salt, and do not leave "the salt of the covenant of your God" out.

“And you shall season each of your grain offerings with salt. You must not leave the salt of the covenant of your God out of your grain offering; you are to add salt to each of your offerings.”— Leviticus 2:13 (BSB)