Place a large heavy-based pan over high heat & add in the clarified butter, allow to melt.
Add in the diced onions & saute for 3 minutes until golden & translucent, stirring frequently. Add in the garlic & saute for 45 seconds, stirring the whole time.
Pour in the brandy (see notes), reduce for 1 minute, stirring frequently. Add in the cracked black pepper, green peppercorns & beef stock, allow to reduce for a further 1 minute, or until reduced by half.
Add in the chopped parsley, thyme & thickened cream, season to taste. Give the sauce a good mix & bring to a simmer. Once simmering, reduce the heat to low & allow the sauce to reduce for 10-12 minutes or until thickened (reduced by ½), stirring frequently.
Once reduced, remove the sprigs of thyme & serve. Dig in.