I've frozen a lot of briskets for reheating later on. Trust me they come out better if you leave them whole. I have a full set of instructions I send along with the brisket here are some snips you might find helpful. I also include photos on how to slice against the grain, but you already know about that.....
PREPARATION FOR FREEZING – Following smoking your brisket was wrapped in a triple layer of foil (with the fat cap up) and several ounces of the brisket mop was added to the pouch before sealing. Then it was rested for a couple of hours in an insulated cooler. This sort of winds down the cooking process. Following this, the entire foil pouch was placed in the freezer.
THAWING – Big pieces of meat reheat better than slices. Here is the plan I think will work best for you. First it will need a long slow thaw in the refrigerator. I took yours out of the freezer late Monday night and placed it the fridge. On Thursday the internal temperature was 31°. This should help you plan ahead for your cooking day.
REHEATING – Remove the brisket from the foil pouch and place it fat cap up in an aluminum roasting pan. Take any of the beef jelly that is in the foil and add that under the brisket in the pan. You also need to add several ounces of the brisket mop I sent along to the pan before the reheat. Be sure to warm it up in the microwave before adding it to the pan. With the fat cap up, the meat side will be resting in some juices which will help to keep it moist. Cover the pan with foil. Set the oven temp to 250°. I’m thinking it will take 1-1/2 to 2 hours total for the reheat. After heating for 1 hour, open the foil and check the internal temperature in a couple of locations. Have more heated mop ready and add some if needed, then reseal the foil cover. (If you are careful, you could turn it over in the pan at the 1 hour mark. I’ll leave this decision up to you) Monitor the internal temp of the meat during the rest of the reheat…..you are shooting for 140°-150° internal temperature. Don’t go much higher or you will risk driving some of the moisture out. Now it should be ready to slice. (About the only other option you could have would be to remove the foil and use the broiler to crisp up the fat side a little. Be VERY careful so you don’t burn it. I’ll leave this decision up to you)
Be sure and save the pan juices, they can be added to the BBQ sauce or served by themselves as a second table sauce. Some can be mixed into another sauce to give you three options for table sauces.
SLICING – You need to plan on slicing right before serving. Brisket will dry out really fast after it is sliced. I would get a volunteer to man the cutting board and slice to order when people move through the serving line. Have the meat at the end of the serving line if possible. Also, slicing is critical to do in the right direction, which is against the grain.
If the first direction of slicing is not correct, just rotate the brisket and take another test slice. When you put it on the cutting board, put the fat cap down. Start with slices 1/4” thick, if they are a little tough slice them thinner. If they fall apart, slice a little thicker.
