Rub both sides of each of the steaks with some cooking oil. Season steaks with salt and pepper on both sides.
Heat a large frying pan or grill pan over high heat. When very hot, add the steaks. Cook both sides until desired doneness - timing depends on thickness of steak. Generally, for 1-inch thick steak, I grill 5 minutes per side then check with meat thermometer (145F is medium-rare, 160F is medium). Remove steaks to a plate and tent with tin foil to rest.
Return the same frying pan to the stove, heat the butter over medium heat. When the butter starts bubbling, add the onions and saute until the onions are soft, translucent and just slightly golden. Take care not to burn the onions by keeping your heat on medium or even medium-low. This will take about 3 minutes. Add the garlic and saute for an additional minute until the garlic is fragrant.
Turn your heat to medium-high and add the mushrooms all at once. Immediately start tossing so that the onion/garlic/butter mixture is evenly distributed amongst the mushrooms. Saute the mushrooms until they are browned and softened, about 3-5 minutes depending on the thickness and size of your mushrooms.
Season with salt, pepper and nutmeg. Pour in the whiskey and the balsamic vinegar and let the mixture bubble a bit to burn off some of the alcohol. Turn the heat down just a bit and add in the heavy cream. Let it bubble for another 30 seconds and then taste to adjust the salt. Stir in the freshly minced herbs and pour over just-grilled steak.