Cut the rump steak or fillet into thin strips about 1 cm wide.
Heat the 4 tablespoons of butter over medium-high heat in a large pan. Fry the beef strips for 3–4 minutes, stirring frequently, until browned on all sides. Transfer to a plate and keep warm.
In the same pan, dice the onion and fry over medium heat for 3 minutes until translucent.
Pour in the red wine, scraping up the browned bits from the bottom of the pan. Let it simmer for 2–3 minutes until reduced by about half.
Stir in the tomato paste (roughly 10 cm squeezed from the tube).
Add the broth, green peppercorns, and cream. Bring to a boil over medium-high heat.
Use an immersion blender to smooth the sauce until well combined.
Return the beef to the pan and simmer for 2–3 minutes until heated through.