Use the pan in which you cooked the steaks, remove any remaining fat with kitchen paper - don't wipe out the crusty bits.
Add the butter and shallots to the pan, allow it to froth and add the green peppercorns.
Partially squash some of them into the butter using the back of a wooden spoon, and then stir in the whisky and garlic (there is no real need to flame the alcohol).
Allow to combine for a few moments and then whisk in the mustard and cream until all is smooth.
Simmer gently - continuing to whisk occasionally - until the copious amount of bubbles begin to reveal an increasingly brown tinge, and the sauce is a creamy consistency, perfect to coat a steak nicely.