The flat is the leanest part of the brisket, so most prone to dying out. A foolproof way would be to season & lay on bed of sliced onions/root vegetables. Fill with enough stock to cover the lower third of the brisket (250-400ml). Cover & check back in 5-6 hrs. Remove cover to reduce the liquid & top up with stock or water of it dries out. Put jt in the oven at 110⁰C & cook slowly for 8 hrs. This is essentially what people did before crock pots & slow cookers.