Spread a generous amount of onion soup mix and oil on brisket

wrap in foil, secure sides so drippings stay inside the foil.

bake in 350f for at least 2 hours.

remove from foil, reserve drippings, let cool, slice to thin slices and arrange slices in a deep serving dish

saute onion in a little oil 

add mushroom soup mix

water

salt

pepper

paprika

sliced mushrooms

reserved dripping

simmer until thickens to a nice sauce

pour over sliced brisket

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Note: Alternatively, instead of baking it can be cooked in an Instapot. The pressure cooking or the slow cooking function may be good.