Sprinkle the surface of the steaks with ground peppercorns, olive oil and soy sauce. Let stand at room temperature for 30 minutes.
Preheat the oven to 220 C / Gas 7.
Heat a cast iron pan over medium-high heat. Place 2 tablespoons butter in the pan and allow it to melt completely and begin to foam. Cook steaks in the hot pan until browned, 2 to 3 minutes per side. Place mushrooms around steaks and stir to coat.
Transfer pan to the preheated oven and cook until steaks are firm and reddish-pink and juicy in the centre, 6 to 8 minutes. An instant-read thermometer inserted into the centre should read 57 C. Remove steaks from the pan and allow to rest.
Return pan to medium heat and add remaining 1 tablespoon butter and garlic. Cook, stirring constantly, until garlic is fragrant, about 2 minutes. Pour in whisky carefully, away from any heat source; add the cream. Allow to simmer until slightly reduced, about 2 minutes. Serve sauce over steaks.