Learn Finnish: Tarjoa ruokaa, jossa on korkeintaan viisi grammaa suolaa. - Serve food that has at most five grams of salt.

Yes, you could say enintään viisi grammaa. Both mean "at most" or "maximum". Korkeintaan comes from korkea (high) and literally means "at its highest", while enintään is related to eniten (most). In contexts of measurement like this, they are completely interchangeable.

English says "that has at most five grams". Why does Finnish use jossa on ("in which is") instead of the possessive jolla on?

In Finnish, when an inanimate object (like food, a building, or a container) contains something as an ingredient or component, you use the inessive case (-ssa/-ssä) rather than the adessive (-lla/-llä). So ruoka, jossa on suolaa literally means "food in which there is salt". You generally reserve jolla on for actual ownership by people or animals.

The verb tarjoa is an imperative command. Shouldn't the object ruokaa be in the basic nominative form?

Normally, total objects in imperative sentences do drop their accusative -n ending and look like the nominative (for example, Osta omena! - Buy an apple!). However, ruoka here is an indefinite mass noun. Because you are offering an unspecified amount of food rather than one specific, whole item, the object must stay in the partitive case.

Why do we have a double partitive here with grammaa suolaa?