When it come to offal the most important thing is to get it from a reputable source - ahem you are in the right place. Organs are filters for the body and as such are greatly affected by what the animal is fed and what medication is used.
I suspect that if you are looking for beef Liver you already have something in mind for them. My gran does a fatty bacon, sweet browned onion and liver dish that is absolutely delicious. Don't over fook them, unless you have a more leathery use for them like lining old flip-flops. They are great flash fried and then rested in a brandy, pepper and cream sauce.
It is one of those things that is best defrosted slowly in the fridge and cooked from room temperature.