It also gives as a possible translation "Grill the meat well". I thought "grill" was a different verb.
How do we know this is imperative? Is the phrase different to 'he/she/it roasts the meat well'?
No,it's not different. That should also be accepted. However, i think this sentence matches better as imperative.
Is this how one would order meat to their liking (well done, medium, rare, etc), or are they just telling someone to do a good job cooking it?