Preheat oven to 375F. Season the Porterhouse steak on both sides with salt and pepper. Let steak rest at room temperature for 15 minutes. Rub both sides of the Porterhouse steak with cooking oil.
Heat a oven-safe pan (cast iron preferred) over high heat. When very hot, add the Porterhouse steak to the pan and let cook for 5 minutes. Flip steak and cook an additional 5 minutes. Place entire pan into oven to cook for 5-7 minutes, timing depends on thickness of steak and desired temperature.
For 1-inch thick steak, I recommend 5 minutes and then check internal temperature of the steak.
For 1 1/2 inch Porterhouse steaks, I recommend checking temperature at the 7 minute mark.125-130F = rare130-140F = medium rare140-150F = medium150-155F = medium well160-212F = well-done
While the steak is the oven, let's cook the mushroom sauce. Heat a saute pan over medium-high heat with the butter. When the butter starts bubbling, add in the mushrooms and the onions. Stir and cook until onions are fragrant and softened.
Pour in the chicken broth, mustard, whiskey and season with salt and pepper to taste. Turn heat to low and let simmer for 2 minutes. Just before serving, pour over the steak.