Soak the liver in well-salted water for 30-60 minutes. Remove and rinse under running water and pat dry.
Cut the liver into slices on a bias. Lay them on a plate and season with salt and pepper.
Heat a skillet over medium-high heat with oil until you can see the heat coming off of it. Sear the liver slices on both sides, approximately 1 minute on each side. You want them medium rare at this point. If your pan is too small to fit all of the liver in one batch, cook the liver in batches so you don't crowd the pan.
Remove all of the liver slices to a plate, add more oil to the skillet and saute the onions and mushrooms until they are soft. Season with salt and pepper as you go.
Return the liver slices to the pan, add the vermouth and let simmer for a few minutes. Then add the whiskey. Light it on fire (flambée) with a match and stand back. Once the alcohol has cooked off and the flames have subsided, add the cream. Cook for a few more minutes, taste and adjust the seasoning. Serve immediately.