Two hours before you plan to start cooking, trim the beef of fat and, cutting against the grain, slice the beef into pieces of around 2cm x 5cm.
Transfer the beef to a sealed container and sprinkle 1/4 teaspoon of salt, a tablespoon of allspice onto the beef, rubbing it into the meat to ensure it is evenly distributed, then refrigerate for two hours.
Remove the beef from the fridge half an hour before you plan to cook to bring it to room temperature.
In a cast iron skillet or large pan, over medium heat, gently fry the onion slices in a tablespoon each of butter and olive oil until translucent and soft, then transfer the onion to a large bowl.
To the same skillet or pan, add another tablespoon each of butter and olive oil, turn the heat to medium-high, add the mushrooms, sauté until they begin to brown and their juices are released, add half the black pepper and paprika, and continue to sauté until soft, then transfer the mushrooms to the same plate as the onions.
In a wok* over high heat, add a tablespoon of olive oil and heat until shimmering, add the beef pieces and stir-fry on high heat for a minute or two until brown, remove from the heat and transfer to a plate to rest for at least 5 minutes. (*If you don't have a wok, use the same skillet or pan you cooked the onions and mushrooms in, wiping the pan first with a paper kitchen towel).
To the skillet or pan you cooked the onions and mushrooms in, over low heat, add the cream, remaining black pepper and paprika, stir until warm, slowly add the beef stock, stirring to combine. Return the onions and mushrooms to the pan, add the mustard, stir to combine, and simmer until the sauce reduces and thickens.
Add the beef pieces and juices to the pan, then the fish sauce and sour cream, stir to combine and taste, adding a little more salt if needed (sometimes I add more spice at this point), and simmer for five minutes or so.
Serve on individual plates, garnished with fresh dill, with sides of crispy shoestring fries, mashed potato or potato gratin, gherkins, and additional dollops of sour cream.