Oven Vs Slow Cooker: Which Is Better For Reheating Brisket? - Tasting Table
Briefly

I don’t recommend using a slow cooker to reheat brisket. Because of the size and shape of the slow cooker, you’ll have to stack the brisket slices on top of each other, which will prolong the preheat time. Instead, you can adjust the temperature more precisely in the oven.
Slow cookers force the slices of brisket to be piled up in the smaller space, making it hard for heat to circulate. This leads to longer cooking times and can cause nicely sliced brisket to transform into shredded beef.
Covering the pan with a layer of foil helps to keep the brisket juicy, allowing the meat to warm through without drying out.
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