My bet is Delis get them precooked. But yes, cook following package directions, and then cool, chil in fridge. THis year I'm buying two cause one just wasn't enough last year, it was SOOOOOOOOOOO good!
Yes, deli's get them precooked. Also, why the deli Corned Beef looks so pretty is that they use a trimmed Bottom or Eye Round, instead of the fatty Brisket.
Ditto to what every one else has said about slicing the Corned Beef while it's cold and against the grain. And Corned Beef really doesn't need to be cooked past 190° or 195° internal to be tender, BUT slicing it against the grain will help to make it more tender.
Buy three Corned Beef's this year and make your own "Pastrami" per the instructions below. Not 'true' Pastrami, but I call it "Lazy Mans Pastrami" and it's pretty darn good and easy to make.
Here is a store bought Corned Beef Brisket that has the fat trimmed off, a cold water bath for 12 hours (to help leech some of the salt, water was changed every 4 hours) and seasoned with garlic powder then a dusting of fresh ground pepper.
Onto a 250° indirect grill with some cherry and hickory smoke.
After approx 5 hours, the temp was up to 188°. It was pulled off the grill, allowed to cool and then wrapped in the fridge over night.
Here we are cut in half....purdy, ain't it?
Onto the slicer and sliced thin
against the grain.
This was some mighty fine "Pastrami"
Rye, Swiss, mustard and some pickles